feat(subgroups): schema 21 -- a parentage is a row, so a room can have two

Phase 2 of docs/subgroups-multiple-parents.md, the storage. Still dark: a lineage of
two now verifies and is filed, and nothing in the app creates one until the adoption
flow lands.

`GroupKeyState` carried `parentChatRoomId` and `birthCertificateJson` because a group
could only be founded with a parent, never adopted by a second. Those are
`GroupKeyStateParent` now, one row per parent, and nothing about what they mean
changed in the move -- including that the parent side is never a foreign key onto
`ChatRoom`, for the three reasons that entity's comment now carries.

**The primary key is the rule.** `(chatRoomId, parentChatRoomId)` is "one certificate
per child per parent", which `SubgroupLineage.of` already refuses to break; the key is
what makes it a property of the store rather than of a reader remembering. The cascade
points at `GroupKeyState`, which is the one cascade here that is wanted: a lineage must
not outlive the state that claimed it, or a room recreated at the same derived id would
inherit a parentage nobody signed for it.

**`replace` rewrites the lineage rather than merging it, and that is why it is a
transaction.** A key state is replaceable and the newest wins *wholesale*, so the set
of parents it names is the set. That is what makes a disavowal expressible at all -- a
room drops a parent by re-announcing without it -- and merged rows would leave the
dropped parent surviving the statement that dropped it. Deleting before inserting
inside one transaction also stops a reader seeing a state beside somebody else's
parents: were the writes separate, a concurrent read could land between them and get a
state with the old lineage, which looks exactly like a true answer. An older state
still changes nothing, now including its parents.

**`stateFrom` returns `LocalGroupKeyState`, because a state and its lineage are one
signed statement.** `lineageOf` checks every parent and drops the whole state if any
one certificate fails -- the all-or-nothing rule generalised, for the reason it was
written: keeping the provable half would file a group as having one parent when its own
members see two. Each link is checked on its own, which is what lets a lineage be
assembled a parent at a time, and there is a comment where somebody will look for the
check that is deliberately absent: nothing requires the certificates to agree on the
set. An earlier certificate cannot name a later co-parent, so requiring agreement would
make every adoption cost a fresh quorum in every group that had already certified the
room. The child's key state is the sole authority.

**`ChatRoom.parentChatRoomId` is dropped, unread.** It was a cache of the verified
value, written by `MarmotGroupCreation` and read in production by nothing at all -- the
live answer went through `GroupKeyState`. Keeping it would mean a column holding *one*
of several parents, which is worse than unused: a cache that is wrong. Its one query,
`ChatRoomDao.observeByParentChatRoomId`, had only tests for callers and goes with it;
the comment left in its place says where the two live answers are. `create`'s
`parentChatRoomId` parameter threaded through four files to reach the write that is now
gone, so it goes too rather than sitting dead.

`MIGRATION_20_21` creates, copies, then drops -- the shape MIGRATION_9_10 established,
and for the same reason it gives: `DROP TABLE` fires cascades and would delete the rows
the migration had just written, so `ALTER TABLE ... DROP COLUMN` is used instead.
`BundledSQLiteDriver` is pinned on every platform, so 3.35 is ours rather than the
host's. The `WHERE` takes both halves of a parentage: a half row should not exist, since
`stateFrom` has always dropped such a state rather than filing it, but this is the one
moment the data is reshaped rather than re-derived and a half row copied forward would
become a parent whose certificate is the string "null". Better to leave it behind -- the
state keeps saying what the room signs with, and loses a lineage it could never prove.

`parentOf` is `parentsOf` and returns a list, through the repository to
`ChatRoomDetailUIState`; the parent card becomes one card per parent, keyed, because
they are a set rather than a path -- neither is "the" parent and the order says nothing
about which came first.

Thirteen tests in `SubgroupDaoJvmTest`, rewritten for the table: the two cascades in
both directions, a child with two parents appearing in both their lists, a
re-announcement that drops a parent and one that clears the lineage entirely, an older
state changing neither the state nor its parents, and the primary key collapsing a
repeat to one row. In `GroupKeyStateTest`, Phase 1's "two parents are dropped" becomes
"two parents are kept, each with its own certificate" -- it needed a third real group,
since the earlier draft's second parent was the child certifying itself -- beside a new
case where one bad certificate of two drops the whole state.

444 common tests, 937 jvm tests, `m3Audit` meets every budget.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

composeApp/src/commonMain/kotlin/press/mantra/compose/database/MantraDatabase.kt

@@ -74,6 +74,7 @@ import press.mantra.compose.database.model.GiftWrapMessage
 import press.mantra.compose.database.model.GiftWrapPayload
 import press.mantra.compose.database.model.GiftWrapSeal
 import press.mantra.compose.database.model.GroupKeyState
+import press.mantra.compose.database.model.GroupKeyStateParent
 import press.mantra.compose.database.model.GroupSignedEvent
 import press.mantra.compose.database.model.InReplyToRelation
 import press.mantra.compose.database.model.MantraArtifact
@@ -139,6 +140,7 @@ val GENESIS_AT = Instant.fromEpochMilliseconds(1231006505000L)
         GiftWrapSeal::class,
         GiftWrapPayload::class,
         GroupKeyState::class,
+        GroupKeyStateParent::class,
         GroupSignedEvent::class,
         InReplyToRelation::class,
         MantraArtifact::class,
@@ -177,7 +179,7 @@ val GENESIS_AT = Instant.fromEpochMilliseconds(1231006505000L)
         UnsignedNostrEvent::class,
         Zap::class
     ],
-    version = 20,
+    version = 21,
     autoMigrations = [
         // v2 only adds the DkgSession/DkgParticipantMessage tables, so Room can
         // generate the migration itself — nothing existing changes shape.
@@ -321,6 +323,15 @@ val GENESIS_AT = Instant.fromEpochMilliseconds(1231006505000L)
         // which is what they were, and any identity may drain them. The broadcast
         // queue deliberately gets no owner: a signed event is anyone's to carry.
         AutoMigration(from = 19, to = 20),
+        // v21 moves a room's parentage off two GroupKeyState columns onto the new
+        // GroupKeyStateParent table, so a room can be the child of more than one
+        // group -- founded with one and adopted by another later. It copies
+        // before it drops, which no AutoMigration can express, and it drops
+        // ChatRoom.parentChatRoomId as well: that column was a cache of the
+        // verified value which nothing in production ever read, and keeping it
+        // would mean a column holding *one* of several parents, which is worse
+        // than unused. See MIGRATION_20_21 and
+        // docs/subgroups-multiple-parents.md.
     ]
 )
 @ColumnTypeConverters(MantraConverters::class)

composeApp/src/commonMain/kotlin/press/mantra/compose/database/builder/PlatformDatabaseBuilder.kt

@@ -3,6 +3,7 @@ package press.mantra.compose.database.builder
 import androidx.room3.RoomDatabase
 import androidx.sqlite.driver.bundled.BundledSQLiteDriver
 import press.mantra.compose.database.migrations.MIGRATION_3_4
+import press.mantra.compose.database.migrations.MIGRATION_20_21
 import press.mantra.compose.database.migrations.MIGRATION_9_10
 import press.mantra.compose.database.migrations.MIGRATION_13_14
 import press.mantra.compose.database.migrations.MIGRATION_14_15
@@ -19,13 +20,21 @@ fun getRoomDatabase(
     builder: RoomDatabase.Builder<press.mantra.compose.database.MantraDatabase>
 ): press.mantra.compose.database.MantraDatabase {
     return builder
-        // Everything else Room generates itself. These four move data rather than
+        // Everything else Room generates itself. These five move data rather than
         // only changing shape, which an AutoMigration cannot express: 3->4 rewrites
         // chat rows, 9->10 copies a session's per-event columns onto the items
         // table before dropping them, 13->14 renames a column and rewrites the
-        // chat rows that named it the old way, and 14->15 adds a column and deletes
-        // the chat rows written for messages from before this device joined.
-        .addMigrations(MIGRATION_3_4, MIGRATION_9_10, MIGRATION_13_14, MIGRATION_14_15)
+        // chat rows that named it the old way, 14->15 adds a column and deletes
+        // the chat rows written for messages from before this device joined, and
+        // 20->21 copies each room's single parentage onto the table that can hold
+        // several before dropping the columns it came from.
+        .addMigrations(
+            MIGRATION_3_4,
+            MIGRATION_9_10,
+            MIGRATION_13_14,
+            MIGRATION_14_15,
+            MIGRATION_20_21
+        )
         .setDriver(BundledSQLiteDriver())
         .setQueryCoroutineContext(Dispatchers.IO)
         .build()

composeApp/src/commonMain/kotlin/press/mantra/compose/database/dao/ChatRoomDao.kt

@@ -66,25 +66,14 @@ interface ChatRoomDao {
     )
     fun observeChatRoomListByUserPublicKey(userPublicKey: String): Flow<List<LocalChatRoom>>
 
-    /**
-     * The subgroups of [parentChatRoomId] this device actually holds rooms for.
-     *
-     * Deliberately not the answer to "what subgroups does this group have" --
-     * that is `SubgroupManager.subgroupsOf`, which reads the parent's signed
-     * certificates and so can name a child whose room has not been created, or
-     * was created for somebody else. This names the ones there is something to
-     * open.
-     *
-     * Soft-deleted rooms are excluded like everywhere else: a room the user has
-     * cleared away should not reappear because its parent lists it.
-     */
-    @Transaction
-    @Query(
-        CHAT_ROOM_WITH_LAST_MESSAGE +
-            "WHERE ChatRoom.parentChatRoomId = :parentChatRoomId AND ChatRoom.deletedAt IS NULL " +
-            ORDER_BY_LAST_ACTIVITY
-    )
-    fun observeByParentChatRoomId(parentChatRoomId: String): Flow<List<LocalChatRoom>>
+    // "The subgroups of this room" used to be a column filter here, over a
+    // ChatRoom.parentChatRoomId that production never read -- the live answer
+    // went through GroupKeyState, and SubgroupManager.subgroupsOf reads the
+    // parent's own certificates, which can name a child whose room this device
+    // does not have. The column is gone at v21, because a room can have more
+    // than one parent and a single column would have held one of them. The
+    // rooms a parent's children occupy are reached through
+    // GroupKeyStateDao.getByParentChatRoomId and resolved from there.
 
     @Upsert
     suspend fun upsert(chatRoom: ChatRoom)

composeApp/src/commonMain/kotlin/press/mantra/compose/database/dao/GroupKeyStateDao.kt

@@ -6,6 +6,8 @@ import androidx.room3.Transaction
 import androidx.room3.Upsert
 import kotlinx.coroutines.flow.Flow
 import press.mantra.compose.database.model.GroupKeyState
+import press.mantra.compose.database.model.GroupKeyStateParent
+import press.mantra.compose.database.model.intermdiate.LocalGroupKeyState
 
 @Dao
 abstract class GroupKeyStateDao {
@@ -15,6 +17,18 @@ abstract class GroupKeyStateDao {
     @Query("SELECT * FROM GroupKeyState WHERE chatRoomId = :chatRoomId")
     abstract fun observeByChatRoomId(chatRoomId: String): Flow<GroupKeyState?>
 
+    /**
+     * A state with the groups that have certified it, which is how a lineage
+     * should always be read -- the two are one signed statement.
+     */
+    @Transaction
+    @Query("SELECT * FROM GroupKeyState WHERE chatRoomId = :chatRoomId")
+    abstract suspend fun getLocalByChatRoomId(chatRoomId: String): LocalGroupKeyState?
+
+    @Transaction
+    @Query("SELECT * FROM GroupKeyState WHERE chatRoomId = :chatRoomId")
+    abstract fun observeLocalByChatRoomId(chatRoomId: String): Flow<LocalGroupKeyState?>
+
     /** Every room that signs with one ceremony's key. One, today. */
     @Query("SELECT * FROM GroupKeyState WHERE dkgSessionId = :dkgSessionId")
     abstract suspend fun getByDkgSessionId(dkgSessionId: String): List<GroupKeyState>
@@ -23,26 +37,52 @@ abstract class GroupKeyStateDao {
      * Every subgroup of [parentChatRoomId] this device holds a verified state
      * for.
      *
-     * Verified is the whole of what these rows are: a state only reaches the
-     * table with a parent on it once its birth certificate has been checked
-     * against that parent's signature, so this is a list of children the parent
-     * really did certify rather than a list of rooms claiming a parent.
+     * Verified is the whole of what these rows are: a parentage only reaches the
+     * table once its birth certificate has been checked against that parent's
+     * signature, so this is a list of children the parent really did certify
+     * rather than a list of rooms claiming a parent.
      *
      * It is not the whole list. A subgroup whose room this device does not have
      * leaves no state here at all, and the certificates in `GroupSignedEvent` are
      * the fuller answer -- see `SubgroupManager.subgroupsOf`.
+     *
+     * A child with two parents appears in both parents' answers, which is the
+     * point of the join: the row that makes it this parent's child says nothing
+     * about who else certified it.
      */
-    @Query("SELECT * FROM GroupKeyState WHERE parentChatRoomId = :parentChatRoomId ORDER BY announcedAt DESC")
-    abstract suspend fun getByParentChatRoomId(parentChatRoomId: String): List<GroupKeyState>
+    @Transaction
+    @Query(
+        "SELECT GroupKeyState.* FROM GroupKeyState " +
+            "JOIN GroupKeyStateParent ON GroupKeyStateParent.chatRoomId = GroupKeyState.chatRoomId " +
+            "WHERE GroupKeyStateParent.parentChatRoomId = :parentChatRoomId " +
+            "ORDER BY GroupKeyState.announcedAt DESC"
+    )
+    abstract suspend fun getByParentChatRoomId(parentChatRoomId: String): List<LocalGroupKeyState>
 
-    @Query("SELECT * FROM GroupKeyState WHERE parentChatRoomId = :parentChatRoomId ORDER BY announcedAt DESC")
-    abstract fun observeByParentChatRoomId(parentChatRoomId: String): Flow<List<GroupKeyState>>
+    @Transaction
+    @Query(
+        "SELECT GroupKeyState.* FROM GroupKeyState " +
+            "JOIN GroupKeyStateParent ON GroupKeyStateParent.chatRoomId = GroupKeyState.chatRoomId " +
+            "WHERE GroupKeyStateParent.parentChatRoomId = :parentChatRoomId " +
+            "ORDER BY GroupKeyState.announcedAt DESC"
+    )
+    abstract fun observeByParentChatRoomId(parentChatRoomId: String): Flow<List<LocalGroupKeyState>>
+
+    /** The parents of one room, as rows. Prefer [getLocalByChatRoomId]. */
+    @Query("SELECT * FROM GroupKeyStateParent WHERE chatRoomId = :chatRoomId")
+    abstract suspend fun getParentsByChatRoomId(chatRoomId: String): List<GroupKeyStateParent>
 
     @Upsert
     abstract suspend fun upsert(groupKeyState: GroupKeyState)
 
+    @Upsert
+    abstract suspend fun upsertParents(parents: List<GroupKeyStateParent>)
+
+    @Query("DELETE FROM GroupKeyStateParent WHERE chatRoomId = :chatRoomId")
+    abstract suspend fun deleteParentsByChatRoomId(chatRoomId: String)
+
     /**
-     * Files a state, keeping the newest one per room.
+     * Files a state and its whole lineage, keeping the newest one per room.
      *
      * This is where the event's replaceable semantics actually happen. Relays
      * never see a `GroupKeyStateEvent` -- it lives inside a Marmot group and is
@@ -56,13 +96,36 @@ abstract class GroupKeyStateDao {
      * state agree by construction, since both derived it from the room they are
      * standing in.
      *
-     * Returns the state now on file.
+     * ### The lineage is replaced, not merged, and that is the whole reason this
+     * is a transaction
+     *
+     * A key state is replaceable and the newest wins **wholesale**, so the set of
+     * parents it names is the set -- not an addition to whatever was there. That
+     * is what makes a disavowal expressible at all: a room drops a parent by
+     * re-announcing without it, and if these rows were merged rather than
+     * rewritten the dropped parent would survive the statement that dropped it.
+     *
+     * Deleting before inserting inside one transaction is also what stops a
+     * reader seeing a state beside somebody else's parents. Were the two writes
+     * separate, a concurrent `getLocalByChatRoomId` could land between them and
+     * read a state with no lineage, or with the old one -- both of which look
+     * exactly like a true answer.
+     *
+     * Returns the state now on file, with the lineage now on file beside it.
      */
     @Transaction
-    open suspend fun replace(groupKeyState: GroupKeyState): GroupKeyState {
+    open suspend fun replace(
+        groupKeyState: GroupKeyState,
+        parents: List<GroupKeyStateParent> = emptyList(),
+    ): LocalGroupKeyState {
         val known = getByChatRoomId(groupKeyState.chatRoomId)
 
-        if (known != null && known.announcedAt >= groupKeyState.announcedAt) return known
+        if (known != null && known.announcedAt >= groupKeyState.announcedAt) {
+            return LocalGroupKeyState(
+                keyState = known,
+                parents = getParentsByChatRoomId(known.chatRoomId)
+            )
+        }
 
         val stamped = groupKeyState.copy(
             createdAt = known?.createdAt ?: groupKeyState.createdAt,
@@ -70,6 +133,13 @@ abstract class GroupKeyStateDao {
         )
         upsert(stamped)
 
-        return stamped
+        // The state's own row has to exist before its parents', because they
+        // reference it. Room turns foreign keys off around a migration but not
+        // around a transaction, so the order here is load-bearing rather than
+        // tidy.
+        deleteParentsByChatRoomId(stamped.chatRoomId)
+        if (parents.isNotEmpty()) upsertParents(parents)
+
+        return LocalGroupKeyState(keyState = stamped, parents = parents)
     }
 }

composeApp/src/commonMain/kotlin/press/mantra/compose/database/migrations/SubgroupLineageMigration.kt

@@ -0,0 +1,86 @@
+package press.mantra.compose.database.migrations
+
+import androidx.room3.migration.Migration
+import androidx.sqlite.SQLiteConnection
+import androidx.sqlite.execSQL
+
+/**
+ * Moves a room's parentage off two `GroupKeyState` columns onto
+ * `GroupKeyStateParent`, so a room can be the child of more than one group.
+ *
+ * Not an `AutoMigration`. Room can create the table and it can drop the columns,
+ * but it cannot copy between them, and this migration is entirely about the copy.
+ *
+ * ### Every existing lineage is a set of one
+ *
+ * A group could only be founded with a parent, never adopted by a second, so
+ * every row that carries a parentage carries exactly one. Each becomes one row
+ * in the new table and nothing is lost -- and the rule the new primary key
+ * states, one certificate per child per parent, is already true of all of them.
+ *
+ * Rows with no parentage -- every group made before subgroups, and every
+ * top-level group made after -- contribute nothing, which is what "nobody's
+ * child" looks like in the new shape: no rows rather than a null.
+ *
+ * The `WHERE` takes both halves. A row with one and not the other should not
+ * exist, because `GroupKeyStateManager.stateFrom` has always dropped a state
+ * carrying a half-claim rather than filing it -- but this is the one moment the
+ * data is reshaped rather than re-derived, and a half row copied forward would
+ * become a parent whose certificate is the string "null". Better to leave such a
+ * row behind entirely: the state keeps saying what the room signs with, which is
+ * the part that matters, and loses a lineage it could never have proved.
+ *
+ * ### `ChatRoom.parentChatRoomId` goes with them, unread
+ *
+ * It was a cache of the verified value, written by `MarmotGroupCreation` and
+ * read -- in production -- by nothing at all; the live answer went through
+ * `GroupKeyState`. Keeping it would mean a column holding *one* of several
+ * parents, which is worse than unused: it is a cache that is wrong. So it is
+ * dropped rather than backfilled, and nothing is copied out of it.
+ *
+ * ### Why `DROP COLUMN` rather than a table rebuild
+ *
+ * The usual way to remove columns in SQLite -- create a new table, copy, drop
+ * the old one, rename -- is unsafe here and quietly so. `GroupKeyStateParent`
+ * references `GroupKeyState(chatRoomId)` `ON DELETE CASCADE`, and `DROP TABLE`
+ * fires cascades: with foreign keys enforced it would delete every parent row
+ * this migration had just written. Dropping `ChatRoom` would be far worse, since
+ * most of the schema cascades off it.
+ *
+ * `ALTER TABLE ... DROP COLUMN` avoids the question. It needs SQLite 3.35 and
+ * the columns must be free of indices and constraints -- all three are, and
+ * `getRoomDatabase` pins `BundledSQLiteDriver` on every platform, so the version
+ * is ours rather than the host's. Both tables keep their keys, their foreign keys
+ * and their indices.
+ */
+val MIGRATION_20_21 = object : Migration(20, 21) {
+    override suspend fun migrate(connection: SQLiteConnection) {
+        connection.execSQL(
+            "CREATE TABLE IF NOT EXISTS `GroupKeyStateParent` (" +
+                "`chatRoomId` TEXT NOT NULL, " +
+                "`parentChatRoomId` TEXT NOT NULL, " +
+                "`birthCertificateJson` TEXT NOT NULL, " +
+                "PRIMARY KEY(`chatRoomId`, `parentChatRoomId`), " +
+                "FOREIGN KEY(`chatRoomId`) REFERENCES `GroupKeyState`(`chatRoomId`) " +
+                "ON UPDATE NO ACTION ON DELETE CASCADE )"
+        )
+        connection.execSQL(
+            "CREATE INDEX IF NOT EXISTS `index_GroupKeyStateParent_parentChatRoomId` " +
+                "ON `GroupKeyStateParent` (`parentChatRoomId`)"
+        )
+
+        connection.execSQL(
+            "INSERT OR IGNORE INTO `GroupKeyStateParent` " +
+                "(`chatRoomId`, `parentChatRoomId`, `birthCertificateJson`) " +
+                "SELECT `chatRoomId`, `parentChatRoomId`, `birthCertificateJson` " +
+                "FROM `GroupKeyState` " +
+                "WHERE `parentChatRoomId` IS NOT NULL AND `birthCertificateJson` IS NOT NULL"
+        )
+
+        listOf("parentChatRoomId", "birthCertificateJson").forEach { column ->
+            connection.execSQL("ALTER TABLE `GroupKeyState` DROP COLUMN `$column`")
+        }
+
+        connection.execSQL("ALTER TABLE `ChatRoom` DROP COLUMN `parentChatRoomId`")
+    }
+}

composeApp/src/commonMain/kotlin/press/mantra/compose/database/model/ChatRoom.kt

@@ -112,29 +112,6 @@ data class ChatRoom(
      */
     val chronicleRequestedAt: Instant? = null,
 
-    /**
-     * The group this room is a subgroup of, or null for a room that is nobody's
-     * child.
-     *
-     * A copy of the verified `GroupKeyState.parentChatRoomId`, kept here so the
-     * room list and the room's own screen can answer "whose child is this"
-     * without a second read. It is written as the room is created or adopts its
-     * key state, and only from a state whose certificate passed
-     * `SubgroupBirthCertificateEvent.certifies`.
-     *
-     * **Never a foreign key onto `ChatRoom`.** Every other reference to a room in
-     * this schema cascades, and a self-referential cascade would mean deleting a
-     * parent silently deletes its subgroups -- and then, by their own cascades,
-     * those rooms' messages, participants, key states, signing sessions and
-     * signed events. A user tidying up a group they left would destroy a group
-     * they are still in. `RESTRICT` would be no better: it would make a parent
-     * undeletable while a child row exists, which is a foreign key deciding a
-     * product question. This routinely names a room this device does not have --
-     * see `GroupKeyState.parentChatRoomId` -- so a dangling value is the normal,
-     * expected state.
-     */
-    val parentChatRoomId: HexKey? = null,
-
     override val createdAt: Instant = Clock.System.now(),
     override val updatedAt: Instant = createdAt,
     override val savedAt: Instant = createdAt,

composeApp/src/commonMain/kotlin/press/mantra/compose/database/model/GroupKeyState.kt

@@ -94,41 +94,16 @@ data class GroupKeyState(
     val announcedAt: Instant,
 
     /**
-     * The group this one is a subgroup of, or null for a group that is nobody's
-     * child -- which is every group made before subgroups existed.
+     * A lineage used to live here, as a parent id and a certificate. It is
+     * `GroupKeyStateParent` now, one row per parent, because a room can have
+     * more than one -- see `docs/subgroups-multiple-parents.md`. Nothing about
+     * what it means changed in the move, including that the parent side is never
+     * a foreign key onto `ChatRoom`; that entity's comment carries the argument.
      *
-     * **Not a foreign key, and that is deliberate.** The three tables around this
-     * one cascade off `ChatRoom`, and a self-referential cascade here would mean
-     * deleting a parent room deletes every subgroup's state beneath it. It could
-     * not be `RESTRICT` either: this routinely names a room the device does not
-     * have at all -- a member of a subgroup who was never in its parent holds the
-     * id and nothing else -- so a dangling value is the normal state and readers
-     * resolve it with a lookup allowed to return null.
-     *
-     * Written only from a state whose [birthCertificateJson] passed
-     * `SubgroupBirthCertificateEvent.certifies`, so a row carrying this is a row
-     * where the parent's quorum really did sign for this room. See
-     * `GroupKeyStateManager.stateFrom`, which drops the whole state rather than
-     * keeping a parentage it could not check.
+     * Read the two together through `LocalGroupKeyState`, never apart: a state
+     * and the lineage it claims are one signed statement, and
+     * `GroupKeyStateManager.stateFrom` believes all of it or drops all of it.
      */
-    val parentChatRoomId: HexKey? = null,
-
-    /**
-     * The parent's signed certificate, whole, as JSON -- or null when this group
-     * has no parent.
-     *
-     * The claim itself, where [parentChatRoomId] is only an index into it. Kept
-     * as the event rather than as its signature so that a reader can check it
-     * again from the row alone: a signature plus a rule for rebuilding the event
-     * it covers is a rule that breaks silently the first time the certificate's
-     * shape changes, and a rebuild differing by one byte hashes to an id whose
-     * signature fails and is indistinguishable from a forgery.
-     *
-     * Always set together with [parentChatRoomId]. One without the other is a
-     * state that never reached this table.
-     */
-    val birthCertificateJson: String? = null,
-
     override val createdAt: Instant = Clock.System.now(),
     override val updatedAt: Instant = createdAt,
     override val savedAt: Instant = createdAt,

composeApp/src/commonMain/kotlin/press/mantra/compose/database/model/GroupKeyStateParent.kt

@@ -0,0 +1,86 @@
+package press.mantra.compose.database.model
+
+import androidx.room3.Entity
+import androidx.room3.ForeignKey
+import androidx.room3.Index
+import com.vitorpamplona.quartz.nip01Core.core.HexKey
+
+/**
+ * One group that has certified a room as its child, and the signature saying so.
+ *
+ * A row per parent, because a room can have more than one: it is founded with
+ * one and may be adopted by another later, and an adoption is the parent
+ * certifying plus the child re-announcing its key state with the pair added. See
+ * `docs/subgroups-multiple-parents.md`.
+ *
+ * These rows were two nullable columns on [GroupKeyState] while a lineage could
+ * only ever be one link. Nothing about their meaning changed in the move.
+ *
+ * ### The primary key is the rule
+ *
+ * `(chatRoomId, parentChatRoomId)` -- one certificate per child per parent. A
+ * second entry for a parent already in the set is one parent asked twice and is
+ * refused by `SubgroupLineage.of` before it gets here; the key is what makes
+ * that a property of the store rather than of a reader remembering.
+ *
+ * ### Verified before it is written, every time
+ *
+ * A row exists only where `SubgroupBirthCertificateEvent.certifies` has said
+ * that this parent's quorum really signed for this room -- see
+ * `GroupKeyStateManager.stateFrom`, which drops the **whole** state when any one
+ * parent's certificate fails. So the presence of a row is itself the claim that
+ * the lineage was checked, and there is no state in which some parents are
+ * believed and others are not.
+ *
+ * [birthCertificateJson] is the certificate whole rather than its signature,
+ * so a reader can check it again from the row alone. A signature plus a rule for
+ * rebuilding the event it covers is a rule that breaks silently the first time
+ * the certificate's shape changes: a rebuild differing by one byte hashes to an
+ * id whose signature fails and is indistinguishable from a forgery.
+ *
+ * ### The cascade points at the state, never at the parent's room
+ *
+ * `ON DELETE CASCADE` from [GroupKeyState] is what keeps a lineage from
+ * outliving the state that claimed it -- and it is what lets a re-announcement
+ * that drops a parent actually drop it, since `GroupKeyStateDao.replace`
+ * rewrites these rows inside the same transaction.
+ *
+ * [parentChatRoomId] is deliberately **not** a foreign key onto `ChatRoom`. A
+ * self-referential cascade would mean deleting a parent room deletes its
+ * children's key states, and then by their own cascades those rooms' messages,
+ * participants, signing sessions and signed events -- a user tidying up a group
+ * they left would destroy a group they are still in. `RESTRICT` would be no
+ * better: it would make a parent undeletable while a child row exists, which is
+ * a foreign key deciding a product question. And this routinely names a room the
+ * device does not have at all, since a member of a subgroup need never have been
+ * in its parent, so a dangling value is the normal, expected state and readers
+ * resolve it with a lookup allowed to return null.
+ */
+@Entity(
+    primaryKeys = ["chatRoomId", "parentChatRoomId"],
+    foreignKeys = [
+        ForeignKey(
+            entity = GroupKeyState::class,
+            parentColumns = ["chatRoomId"],
+            childColumns = ["chatRoomId"],
+            onDelete = ForeignKey.CASCADE,
+        )
+    ],
+    indices = [
+        // The composite primary key already indexes chatRoomId as its leftmost
+        // column, so only the other direction needs one: "which rooms has this
+        // group certified", which is how a parent finds the children whose state
+        // this device holds.
+        Index("parentChatRoomId"),
+    ],
+)
+data class GroupKeyStateParent(
+    /** The child: the room whose key state carries this parentage. */
+    val chatRoomId: String,
+
+    /** The parent's room id, which is also the key its certificate verifies against. */
+    val parentChatRoomId: HexKey,
+
+    /** The parent's signed certificate, whole, as JSON. */
+    val birthCertificateJson: String,
+)

composeApp/src/commonMain/kotlin/press/mantra/compose/database/model/intermdiate/LocalGroupKeyState.kt

@@ -0,0 +1,62 @@
+package press.mantra.compose.database.model.intermdiate
+
+import androidx.room3.Embedded
+import androidx.room3.Relation
+import com.vitorpamplona.quartz.nip01Core.core.Event
+import press.mantra.compose.database.model.GroupKeyState
+import press.mantra.compose.database.model.GroupKeyStateParent
+import press.mantra.compose.nostr.subgroup.SubgroupLineage
+import press.mantra.compose.nostr.subgroup.SubgroupParentage
+
+/**
+ * A room's key state with every group that has certified it as a child.
+ *
+ * The two always travel together, because a state and the lineage it claims are
+ * one signed statement: `GroupKeyStateManager.stateFrom` either believes all of
+ * it or drops all of it, so a reader holding the state without the parents would
+ * be holding half of something that has no halves.
+ *
+ * Also the shape `stateFrom` returns. It is a Room read model -- `@Embedded`
+ * over `@Relation`, like `LocalChatRoom` -- and a plain data class besides, so
+ * the pure verifier can build one with no database anywhere near it.
+ */
+data class LocalGroupKeyState(
+    @Embedded
+    val keyState: GroupKeyState,
+
+    @Relation(
+        entity = GroupKeyStateParent::class,
+        parentColumns = ["chatRoomId"],
+        entityColumns = ["chatRoomId"],
+    )
+    val parents: List<GroupKeyStateParent> = emptyList(),
+) {
+    /**
+     * The lineage these rows amount to, or null for a room that is nobody's
+     * child.
+     *
+     * Null also where the rows cannot be read back as a lineage -- a certificate
+     * whose JSON no longer parses, or a repeat the store should have made
+     * impossible. Both are the same answer here, and it is the safe one: a
+     * lineage that cannot be stated is not half-stated, for the reason
+     * `stateFrom` drops a whole state rather than keeping a parentage it could
+     * not check.
+     *
+     * Rebuilt rather than cached, so the canonical order and the no-repeats rule
+     * come from `SubgroupLineage` on every read instead of from whatever order
+     * the rows came back in.
+     */
+    fun lineage(): SubgroupLineage? = SubgroupLineage.of(
+        parents.mapNotNull { parent ->
+            Event.fromJsonOrNull(parent.birthCertificateJson)?.let { certificate ->
+                SubgroupParentage(
+                    parentChatRoomId = parent.parentChatRoomId,
+                    certificate = certificate
+                )
+            }
+        }.takeIf { it.size == parents.size } ?: return null
+    )
+
+    /** The groups that have certified this room, in canonical order. */
+    fun parentChatRoomIds(): List<String> = lineage()?.parentChatRoomIds.orEmpty()
+}

composeApp/src/commonMain/kotlin/press/mantra/compose/database/repository/DatabaseChatRepository.kt

@@ -55,7 +55,6 @@ class DatabaseChatRepository(
         userPublicKey: HexKey,
         keyPair: KeyPair,
         path: List<Long>,
-        parentChatRoomId: HexKey?,
     ): MarmotGroupCreation.Outcome = MarmotGroupCreation.create(
         database = database,
         chatRepository = this,
@@ -66,7 +65,6 @@ class DatabaseChatRepository(
         userPublicKey = userPublicKey,
         keyPair = keyPair,
         path = path,
-        parentChatRoomId = parentChatRoomId,
     )
     override suspend fun observeChatRoomListByPublicKey(publicKey: String): Flow<List<LocalChatRoom>> {
         return database.chatRoomDao().observeChatRoomListByUserPublicKey(publicKey)
@@ -113,11 +111,11 @@ class DatabaseChatRepository(
         emptyList()
     }
 
-    override suspend fun parentOf(chatRoomId: String): HexKey? = try {
-        SubgroupManager.parentOf(database, chatRoomId)
+    override suspend fun parentsOf(chatRoomId: String): List<HexKey> = try {
+        SubgroupManager.parentsOf(database, chatRoomId)
     } catch (e: Throwable) {
-        logger.e("Error reading the parent of $chatRoomId", e)
-        null
+        logger.e("Error reading the parents of $chatRoomId", e)
+        emptyList()
     }
 
     override suspend fun groupSignedEvent(chatRoomId: String, id: HexKey): GroupSignedEvent? = try {

composeApp/src/commonMain/kotlin/press/mantra/compose/database/repository/DatabaseDkgRepository.kt

@@ -5,7 +5,7 @@ import press.mantra.compose.database.model.DkgParticipantMessage
 import press.mantra.compose.database.model.DkgSession
 import press.mantra.compose.database.model.types.DkgRitualStage
 import press.mantra.compose.database.model.FrostSigningSession
-import press.mantra.compose.database.model.GroupKeyState
+import press.mantra.compose.database.model.intermdiate.LocalGroupKeyState
 import press.mantra.compose.database.model.GroupSignedEvent
 import press.mantra.compose.database.model.intermdiate.LocalChatRoom
 import press.mantra.compose.database.model.intermdiate.LocalFrostSigningSession
@@ -110,7 +110,7 @@ class DatabaseDkgRepository(
     ): Flow<List<LocalFrostSigningSession>> =
         database.frostSigningSessionDao().observeSessionsForChatRoom(chatRoomId)
 
-    override fun observeSignedGroupKeyState(dkgSessionId: String): Flow<GroupKeyState?> =
+    override fun observeSignedGroupKeyState(dkgSessionId: String): Flow<LocalGroupKeyState?> =
         database.groupSignedEventDao()
             .observeByKind(GroupKeyStateEvent.KIND)
             .map { signedEvents ->
@@ -132,7 +132,7 @@ class DatabaseDkgRepository(
                 subjectRoomId?.let { GroupKeyStateManager.stateAmong(signedEvents, it) }
             }
 
-    override suspend fun adoptGroupKeyState(chatRoomId: String): GroupKeyState? = try {
+    override suspend fun adoptGroupKeyState(chatRoomId: String): LocalGroupKeyState? = try {
         GroupKeyStateManager.adopt(database = database, chatRoomId = chatRoomId)
     } catch (e: Throwable) {
         // The room is made and usable either way; what it loses is the shortcut

composeApp/src/commonMain/kotlin/press/mantra/compose/managers/GroupKeyStateManager.kt

@@ -9,8 +9,10 @@ import press.mantra.compose.database.MantraDatabase
 import press.mantra.compose.database.model.DkgSession
 import press.mantra.compose.database.model.FrostSigningSession
 import press.mantra.compose.database.model.GroupKeyState
+import press.mantra.compose.database.model.GroupKeyStateParent
 import press.mantra.compose.database.model.GroupSignedEvent
 import press.mantra.compose.database.model.intermdiate.LocalChatRoom
+import press.mantra.compose.database.model.intermdiate.LocalGroupKeyState
 import press.mantra.compose.nostr.frost.GroupKeyStateEvent
 import press.mantra.compose.nostr.subgroup.SubgroupBirthCertificateEvent
 import press.mantra.compose.nostr.subgroup.SubgroupLineage
@@ -122,18 +124,6 @@ object GroupKeyStateManager {
                 "Ceremony ${key.id} has produced no key for $signingRoomId to propose"
             )
 
-        // The event can carry a lineage of any size; the row cannot yet. Until
-        // `GroupKeyState` holds a set, a caller reaching here with two parents
-        // would have one of them silently dropped on the way to the columns, so
-        // it is refused outright rather than half-kept. The tags are already
-        // plural -- see `GroupKeyStateEvent.assembleTags` -- which is what lets
-        // the wire land before the storage does.
-        check(lineage == null || lineage.size == 1) {
-            "Room $signingRoomId cannot yet propose a key state naming " +
-                "${lineage?.size} parents"
-        }
-        val parent = lineage?.parents?.single()
-
         // The room this is about: the group's key walked to the admin path,
         // which is that room's id. It need not exist yet, and in the flow this
         // was written for it does not.
@@ -150,9 +140,7 @@ object GroupKeyStateManager {
             thresholdPublicKey = thresholdPublicKey,
             derivationPath = SharedKeyDerivation.formatPath(path),
             announcedBy = userPublicKey,
-            announcedAt = Instant.fromEpochSeconds(createdAt),
-            parentChatRoomId = parent?.parentChatRoomId,
-            birthCertificateJson = parent?.certificate?.toJson()
+            announcedAt = Instant.fromEpochSeconds(createdAt)
         )
 
         check(state.verifies()) {
@@ -160,10 +148,14 @@ object GroupKeyStateManager {
         }
 
         // Refused here rather than discovered by the group. Every device that
-        // receives this state runs the same check in `parentageOf` and drops the
-        // state when it fails, so proposing one this device would not believe
-        // spends a quorum's attention on a statement nobody will keep.
-        if (parent != null) {
+        // receives this state runs the same checks in `lineageOf` and drops the
+        // state when any of them fails, so proposing one this device would not
+        // believe spends a quorum's attention on a statement nobody will keep.
+        //
+        // Every parent, because the receiving check is all-or-nothing: one bad
+        // certificate in a set of two drops the whole state, so a proposal is
+        // only worth making if every link in it holds.
+        lineage?.parents?.forEach { parent ->
             check(
                 SubgroupBirthCertificateEvent.certifies(
                     event = parent.certificate,
@@ -214,18 +206,19 @@ object GroupKeyStateManager {
         database: MantraDatabase,
         chatRoomId: String,
         innerEvent: Event
-    ): GroupKeyState? {
-        val state = stateFrom(chatRoomId, innerEvent) ?: return null
+    ): LocalGroupKeyState? {
+        val local = stateFrom(chatRoomId, innerEvent) ?: return null
+        val announced = local.keyState.chatRoomId
 
-        if (database.chatRoomDao().findChatRoomById(state.chatRoomId) == null) {
+        if (database.chatRoomDao().findChatRoomById(announced) == null) {
             logger.i(
-                "The group signed a key state for ${state.chatRoomId}, which does not exist " +
+                "The group signed a key state for $announced, which does not exist " +
                     "yet; holding it until it does"
             )
             return null
         }
 
-        return database.groupKeyStateDao().replace(state)
+        return database.groupKeyStateDao().replace(local.keyState, local.parents)
     }
 
     /**
@@ -248,12 +241,15 @@ object GroupKeyStateManager {
      * Idempotent: `replace` keeps the newest state per room, so adopting twice
      * settles on the same row.
      */
-    suspend fun adopt(database: MantraDatabase, chatRoomId: String): GroupKeyState? {
-        val state = signedStateFor(database, chatRoomId) ?: return null
+    suspend fun adopt(database: MantraDatabase, chatRoomId: String): LocalGroupKeyState? {
+        val local = signedStateFor(database, chatRoomId) ?: return null
 
-        logger.i("Room $chatRoomId adopts the key state its group signed at ${state.announcedAt}")
+        logger.i(
+            "Room $chatRoomId adopts the key state its group signed at " +
+                "${local.keyState.announcedAt}, naming ${local.parents.size} parents"
+        )
 
-        return database.groupKeyStateDao().replace(state)
+        return database.groupKeyStateDao().replace(local.keyState, local.parents)
     }
 
     /**
@@ -266,7 +262,7 @@ object GroupKeyStateManager {
      * the ritual screen gates the "create the room" button on, and what [adopt]
      * files once there is a room to file it against.
      */
-    suspend fun signedStateFor(database: MantraDatabase, chatRoomId: String): GroupKeyState? =
+    suspend fun signedStateFor(database: MantraDatabase, chatRoomId: String): LocalGroupKeyState? =
         stateAmong(database.groupSignedEventDao().getByKind(GroupKeyStateEvent.KIND), chatRoomId)
 
     /**
@@ -295,7 +291,7 @@ object GroupKeyStateManager {
      * arrive can ask the same question of each emission -- which is how the
      * ceremony screen knows the group has finished agreeing.
      */
-    fun stateAmong(signedEvents: List<GroupSignedEvent>, chatRoomId: String): GroupKeyState? =
+    fun stateAmong(signedEvents: List<GroupSignedEvent>, chatRoomId: String): LocalGroupKeyState? =
         signedAmong(signedEvents, chatRoomId)?.second
 
     /**
@@ -310,7 +306,7 @@ object GroupKeyStateManager {
     private fun signedAmong(
         signedEvents: List<GroupSignedEvent>,
         chatRoomId: String
-    ): Pair<GroupSignedEvent, GroupKeyState>? =
+    ): Pair<GroupSignedEvent, LocalGroupKeyState>? =
         signedEvents
             .asSequence()
             .filter { it.kind == GroupKeyStateEvent.KIND }
@@ -319,7 +315,7 @@ object GroupKeyStateManager {
             // every room at once, so a state that names no room names nothing.
             .filter { GroupKeyStateEvent.parseChatRoomId(it.tags) == chatRoomId }
             .mapNotNull { signed -> stateFrom(chatRoomId, signed.toEvent())?.let { signed to it } }
-            .maxByOrNull { (_, state) -> state.announcedAt }
+            .maxByOrNull { (_, local) -> local.keyState.announcedAt }
 
     /**
      * The state an event amounts to, or null if it amounts to none.
@@ -358,20 +354,21 @@ object GroupKeyStateManager {
      * names no room at all, and appears in the logs.
      */
     /**
-     * A parentage that was checked, wrapping a value that is allowed to be
+     * A lineage that was checked, wrapping a value that is allowed to be
      * absent.
      *
      * Three outcomes have to be told apart and a bare nullable can only carry
      * two. A state may claim no parent, which is every group made before
-     * subgroups and every top-level group made after; it may claim one and prove
-     * it; or it may claim one it cannot prove, which is not a state with a field
-     * missing but a device asserting a relationship the parent never agreed to.
-     * The third has to stop [stateFrom] rather than be flattened into the first.
+     * subgroups and every top-level group made after; it may claim some and
+     * prove them all; or it may claim some it cannot prove, which is not a state
+     * with a field missing but a device asserting a relationship a parent never
+     * agreed to. The third has to stop [stateFrom] rather than be flattened into
+     * the first.
      *
      * So `null` here means refused, and a present [value] of `null` means none
      * claimed.
      */
-    private class Parentage(val value: SubgroupParentage?)
+    private class Lineage(val value: SubgroupLineage?)
 
     /**
      * The parentage an event actually establishes, or null if it claims one it
@@ -387,12 +384,12 @@ object GroupKeyStateManager {
      * not a state with one field wrong, and half-believing it would file a group
      * as top-level that its own members will see as a subgroup.
      */
-    private fun parentageOf(
+    private fun lineageOf(
         announced: String,
         thresholdPublicKey: HexKey,
         innerEvent: Event
-    ): Parentage? {
-        if (!GroupKeyStateEvent.claimsParentage(innerEvent.tags)) return Parentage(null)
+    ): Lineage? {
+        if (!GroupKeyStateEvent.claimsParentage(innerEvent.tags)) return Lineage(null)
 
         // Both or neither, per parent, and no parent twice -- all of which
         // `parseLineage` decides. A parent named with no certificate is a claim
@@ -410,55 +407,57 @@ object GroupKeyStateManager {
             return null
         }
 
-        // One parent is all a row can hold until `GroupKeyState` carries a set.
-        // Refused rather than truncated: taking the first of two would file a
-        // lineage the group did not sign and leave the device believing a group
-        // has one parent when its own members see two. Nothing in this build
-        // emits a second, so this fires only on a state from a newer one.
-        if (lineage.size > 1) {
-            logger.w(
-                "Key state for $announced claims ${lineage.size} parents " +
-                    "(${lineage.parentChatRoomIds}), which this build cannot file; dropping"
-            )
-            return null
+        // Every parent, or none of them. One bad certificate drops the whole
+        // state rather than the one pair, for the reason the all-or-nothing rule
+        // was written: a state that claims a lineage it cannot back is a device
+        // asserting a relationship somebody never agreed to, and keeping the
+        // provable half would file a group as having one parent when its own
+        // members see two. A set that is partly believed is not a weaker set.
+        lineage.parents.forEach { parentage ->
+            val parentChatRoomId = parentage.parentChatRoomId
+            val certificate = parentage.certificate
+
+            // The parent's own quorum, over this very room's id. Everything that
+            // makes a lineage worth anything is in here -- see
+            // `SubgroupBirthCertificateEvent.certifies`, which needs no lookup,
+            // no key it has to be told, and no knowledge of the other parents.
+            // Each link is checked on its own, which is why a lineage can be
+            // assembled a parent at a time.
+            if (!SubgroupBirthCertificateEvent.certifies(certificate, announced, parentChatRoomId)) {
+                logger.w(
+                    "Key state for $announced carries no certificate by $parentChatRoomId for " +
+                        "that room; dropping"
+                )
+                return null
+            }
+
+            // Belt and braces: the id already derives from this key, and the
+            // certificate's id already derives from the key it names, so two
+            // certificates for one room cannot name two keys. Stated anyway
+            // because the two facts live in different places and a future change
+            // to either should have to notice this.
+            val certifiedKey =
+                SubgroupBirthCertificateEvent.parseThresholdPublicKey(certificate.tags)
+            if (!thresholdPublicKey.equals(certifiedKey, ignoreCase = true)) {
+                logger.w(
+                    "Key state for $announced names $thresholdPublicKey and the certificate " +
+                        "by $parentChatRoomId names $certifiedKey; dropping"
+                )
+                return null
+            }
         }
 
-        val parentage = lineage.parents.single()
-        val parentChatRoomId = parentage.parentChatRoomId
-        val certificate = parentage.certificate
-
-        // The parent's own quorum, over this very room's id. Everything that
-        // makes a lineage worth anything is in here -- see
-        // `SubgroupBirthCertificateEvent.certifies`, which needs no lookup and no
-        // key it has to be told.
-        if (!SubgroupBirthCertificateEvent.certifies(certificate, announced, parentChatRoomId)) {
-            logger.w(
-                "Key state for $announced carries no certificate by $parentChatRoomId for " +
-                    "that room; dropping"
-            )
-            return null
-        }
-
-        // Belt and braces: the id already derives from this key, and the
-        // certificate's id already derives from the key it names, so two
-        // certificates for one room cannot name two keys. Stated anyway because
-        // the two facts live in different places and a future change to either
-        // should have to notice this.
-        val certifiedKey = SubgroupBirthCertificateEvent.parseThresholdPublicKey(certificate.tags)
-        if (!thresholdPublicKey.equals(certifiedKey, ignoreCase = true)) {
-            logger.w(
-                "Key state for $announced names $thresholdPublicKey and its certificate names " +
-                    "$certifiedKey; dropping"
-            )
-            return null
-        }
-
-        return Parentage(
-            SubgroupParentage(parentChatRoomId = parentChatRoomId, certificate = certificate)
-        )
+        // Deliberately not checked: whether the parents know about each other.
+        // An earlier certificate cannot name a later co-parent -- the first
+        // parent signed before the second was a possibility -- so requiring them
+        // to agree on the set would make every adoption cost a fresh quorum in
+        // every group that had already certified this room. The child's key state
+        // is the sole authority on the set; each parent has a veto on being added
+        // and none on who else is in it. See docs/subgroups-multiple-parents.md.
+        return Lineage(lineage)
     }
 
-    fun stateFrom(chatRoomId: String, innerEvent: Event): GroupKeyState? {
+    fun stateFrom(chatRoomId: String, innerEvent: Event): LocalGroupKeyState? {
         val announced = GroupKeyStateEvent.parseChatRoomId(innerEvent.tags) ?: chatRoomId
 
         val thresholdPublicKey = GroupKeyStateEvent.parseThresholdPublicKey(innerEvent.content)
@@ -491,7 +490,7 @@ object GroupKeyStateManager {
             return null
         }
 
-        val parent = parentageOf(announced, thresholdPublicKey, innerEvent) ?: return null
+        val lineage = lineageOf(announced, thresholdPublicKey, innerEvent) ?: return null
 
         val state = GroupKeyState(
             chatRoomId = announced,
@@ -499,9 +498,7 @@ object GroupKeyStateManager {
             thresholdPublicKey = thresholdPublicKey,
             derivationPath = SharedKeyDerivation.formatPath(path),
             announcedBy = innerEvent.pubKey,
-            announcedAt = Instant.fromEpochSeconds(innerEvent.createdAt),
-            parentChatRoomId = parent.value?.parentChatRoomId,
-            birthCertificateJson = parent.value?.certificate?.toJson()
+            announcedAt = Instant.fromEpochSeconds(innerEvent.createdAt)
         )
 
         // Half the trust model, in one line, and the half that does not care who
@@ -514,6 +511,15 @@ object GroupKeyStateManager {
             return null
         }
 
-        return state
+        return LocalGroupKeyState(
+            keyState = state,
+            parents = lineage.value?.parents?.map {
+                GroupKeyStateParent(
+                    chatRoomId = announced,
+                    parentChatRoomId = it.parentChatRoomId,
+                    birthCertificateJson = it.certificate.toJson()
+                )
+            }.orEmpty()
+        )
     }
 }

composeApp/src/commonMain/kotlin/press/mantra/compose/managers/MarmotGroupCreation.kt

@@ -93,10 +93,6 @@ object MarmotGroupCreation {
      * appended to it, because MIP-01 has no field for one and the path is what
      * rebuilds the `TweakCache` a signing session needs.
      *
-     * [parentChatRoomId] is written onto the room when this is a subgroup. It is
-     * the verified value off the child's own key state -- never a claim off a
-     * proposal.
-     *
      * [name] is put under [MarmotGroupName] here rather than trusted from the
      * caller. Every Marmot room on a device is a `#name`, and this is the funnel
      * both flows that make one already go through, so it is the one place the
@@ -112,7 +108,6 @@ object MarmotGroupCreation {
         userPublicKey: HexKey,
         keyPair: KeyPair,
         path: List<Long> = SharedKeyDerivation.MARMOT_ADMIN_GROUP_PATH,
-        parentChatRoomId: HexKey? = null,
         keyPackageLookupTimeout: Long = KEY_PACKAGE_LOOKUP_TIMEOUT
     ): Outcome {
         chatRepository.getChatRoomByIdentifier(groupId)?.let { return Outcome.Existing(it) }
@@ -163,15 +158,6 @@ object MarmotGroupCreation {
             mlsGroupState = group.saveState().encodeTls().toHex()
         ) ?: return Outcome.Failed("Couldn't create the group. Please try again.")
 
-        if (parentChatRoomId != null) {
-            // Written after the room exists rather than passed into its creation,
-            // because `getOrCreateChatRoom` is shared with every other way a room
-            // appears and none of them has a parent to hand it.
-            database.chatRoomDao().upsert(
-                localChatRoom.chatRoom.copy(parentChatRoomId = parentChatRoomId)
-            )
-        }
-
         // The room comes into existence already knowing what it signs with. The
         // group agreed that before any of this ran and this device has been
         // holding the signed statement since; this is simply the first moment

composeApp/src/commonMain/kotlin/press/mantra/compose/managers/SubgroupManager.kt

@@ -410,18 +410,26 @@ object SubgroupManager {
             .sortedByDescending { it.certificate.createdAt }
 
     /**
-     * The group a room is a subgroup of, verified, or null if it is nobody's
-     * child.
+     * Every group that has certified a room as its child, verified, in canonical
+     * order -- empty for a room that is nobody's child.
      *
-     * Off the `GroupKeyState`, which only carries a parent whose certificate was
-     * checked -- see `GroupKeyStateManager.stateFrom`. A room whose state this
-     * device does not hold answers null even when it really is a subgroup, which
-     * is the position a member welcomed in after the founding is in; the
-     * unverified hint in the room's own description is the most they get, and it
-     * must never be written here.
+     * Off `GroupKeyStateParent`, whose rows exist only where the certificate was
+     * checked against that parent's own signature -- see
+     * `GroupKeyStateManager.stateFrom`, which drops the whole state rather than
+     * keeping a lineage it could not check. So every id here is a group that
+     * really did certify this room.
+     *
+     * More than one is the ordinary case now, not a corner: a group is founded
+     * with one parent and may be adopted by another later. The order is the
+     * lineage's own and says nothing about which came first.
+     *
+     * A room whose state this device does not hold answers empty even when it
+     * really is a subgroup, which is the position a member welcomed in after the
+     * founding is in. Nothing unverified may be written here.
      */
-    suspend fun parentOf(database: MantraDatabase, chatRoomId: String): HexKey? =
-        database.groupKeyStateDao().getByChatRoomId(chatRoomId)?.parentChatRoomId
+    suspend fun parentsOf(database: MantraDatabase, chatRoomId: String): List<HexKey> =
+        database.groupKeyStateDao().getLocalByChatRoomId(chatRoomId)?.parentChatRoomIds()
+            .orEmpty()
 
     /**
      * Files a certificate that arrived, or drops it and says why.

composeApp/src/commonMain/kotlin/press/mantra/compose/repository/ChatRepository.kt

@@ -70,14 +70,16 @@ interface ChatRepository {
     suspend fun subgroupsOf(chatRoomId: String): List<SubgroupManager.Subgroup>
 
     /**
-     * The group this room is a subgroup of, verified, or null if it is nobody's
-     * child.
+     * Every group that has certified this room as its child, verified, in
+     * canonical order -- empty if it is nobody's child.
      *
-     * Off the room's `GroupKeyState`, which only carries a parent whose
-     * certificate was checked. A member welcomed in after the founding holds no
-     * state and gets null here even when the room really is a subgroup.
+     * Off `GroupKeyStateParent`, whose rows exist only where the certificate was
+     * checked. More than one is ordinary: a group is founded with one parent and
+     * may be adopted by another later. A member welcomed in after the founding
+     * holds no state and gets an empty list even when the room really is a
+     * subgroup.
      */
-    suspend fun parentOf(chatRoomId: String): HexKey?
+    suspend fun parentsOf(chatRoomId: String): List<HexKey>
 
     /**
      * One event the group signed, as the group signed it, or null when this
@@ -203,7 +205,6 @@ interface ChatRepository {
         userPublicKey: HexKey,
         keyPair: KeyPair,
         path: List<Long> = SharedKeyDerivation.MARMOT_ADMIN_GROUP_PATH,
-        parentChatRoomId: HexKey? = null,
     ): MarmotGroupCreation.Outcome
 
     /**
@@ -279,7 +280,7 @@ interface ChatRepository {
             override suspend fun subgroupsOf(chatRoomId: String): List<SubgroupManager.Subgroup> =
                 emptyList()
 
-            override suspend fun parentOf(chatRoomId: String): HexKey? = null
+            override suspend fun parentsOf(chatRoomId: String): List<HexKey> = emptyList()
 
             override suspend fun groupSignedEvent(
                 chatRoomId: String,
@@ -386,7 +387,6 @@ interface ChatRepository {
                 userPublicKey: HexKey,
                 keyPair: KeyPair,
                 path: List<Long>,
-                parentChatRoomId: HexKey?,
             ): MarmotGroupCreation.Outcome =
                 MarmotGroupCreation.Outcome.Failed("No database")
 

composeApp/src/commonMain/kotlin/press/mantra/compose/repository/DkgRepository.kt

@@ -3,7 +3,7 @@ package press.mantra.compose.repository
 import press.mantra.compose.database.model.DkgParticipantMessage
 import press.mantra.compose.database.model.DkgSession
 import press.mantra.compose.database.model.FrostSigningSession
-import press.mantra.compose.database.model.GroupKeyState
+import press.mantra.compose.database.model.intermdiate.LocalGroupKeyState
 import press.mantra.compose.database.model.GroupSignedEvent
 import press.mantra.compose.database.model.intermdiate.LocalChatRoom
 import press.mantra.compose.database.model.intermdiate.LocalFrostSigningSession
@@ -149,10 +149,10 @@ interface DkgRepository {
      * a finished subgroup with its key state signed, its certificate signed, and
      * no way to create the room -- the lookup found no ceremony at all.
      */
-    fun observeSignedGroupKeyState(dkgSessionId: String): Flow<GroupKeyState?>
+    fun observeSignedGroupKeyState(dkgSessionId: String): Flow<LocalGroupKeyState?>
 
     /** Files the state the group signed for a room that now exists. */
-    suspend fun adoptGroupKeyState(chatRoomId: String): GroupKeyState?
+    suspend fun adoptGroupKeyState(chatRoomId: String): LocalGroupKeyState?
 
     /**
      * Asks the parent's quorum to certify the subgroup this ceremony produced.
@@ -242,10 +242,10 @@ interface DkgRepository {
                 chatRoomId: String
             ): Flow<List<LocalFrostSigningSession>> = flowOf(emptyList())
 
-            override fun observeSignedGroupKeyState(dkgSessionId: String): Flow<GroupKeyState?> =
+            override fun observeSignedGroupKeyState(dkgSessionId: String): Flow<LocalGroupKeyState?> =
                 flowOf(null)
 
-            override suspend fun adoptGroupKeyState(chatRoomId: String): GroupKeyState? = null
+            override suspend fun adoptGroupKeyState(chatRoomId: String): LocalGroupKeyState? = null
 
             override suspend fun proposeBirthCertificate(
                 parentChatRoomId: String,

composeApp/src/commonMain/kotlin/press/mantra/compose/ui/composable/ChatRoomDetailScreen.kt

@@ -259,8 +259,14 @@ fun ChatRoomDetailScreen(
                             // nobody's child, and absent -- rather than guessed --
                             // on a subgroup this device holds no key state for,
                             // since an unverified lineage is not one worth showing.
-                            chatRoomDetailUIState.parentChatRoomId?.let { parentChatRoomId ->
-                                item {
+                            //
+                            // One card per parent, because a room can have more
+                            // than one: founded with one and adopted by another
+                            // later. They are a set rather than a path -- neither
+                            // is "the" parent and the order says nothing about
+                            // which came first -- so each gets an equal row.
+                            chatRoomDetailUIState.parentChatRoomIds.forEach { parentChatRoomId ->
+                                item(key = "parent-$parentChatRoomId") {
                                     Card(
                                         modifier = Modifier.fillMaxWidth(),
                                         onClick = {

composeApp/src/commonMain/kotlin/press/mantra/compose/ui/view/model/ChatRoomDetailViewModel.kt

@@ -81,7 +81,7 @@ class ChatRoomDetailViewModel(
                     groupKeyState = chatRepository.groupKeyState(chatRoomId),
                     signedGroupKeyStateEvent = chatRepository.signedGroupKeyStateEvent(chatRoomId),
                     subgroups = chatRepository.subgroupsOf(chatRoomId),
-                    parentChatRoomId = chatRepository.parentOf(chatRoomId),
+                    parentChatRoomIds = chatRepository.parentsOf(chatRoomId),
                     canAddSubgroup = localChatRoom.chatRoom.mlsGroupState != null &&
                         chatRepository.canSign(chatRoomId),
                 )

composeApp/src/commonMain/kotlin/press/mantra/compose/ui/view/model/DkgRitualViewModel.kt

@@ -607,11 +607,7 @@ class DkgRitualViewModel(
                 },
                 adminPublicKeys = members,
                 userPublicKey = activeUserPublicKey,
-                keyPair = keyPair,
-                // Verified by the time it is written: the key state this room was
-                // gated on carries the certificate, and `stateFrom` refused it
-                // otherwise.
-                parentChatRoomId = subgroupParent
+                keyPair = keyPair
             )
 
             isActionPending.value = false

composeApp/src/commonMain/kotlin/press/mantra/compose/ui/view/state/ChatRoomDetailUIState.kt

@@ -49,7 +49,7 @@ sealed interface ChatRoomDetailUIState {
          * founding holds no state and so sees nothing here, even in a room that
          * really is a subgroup.
          */
-        val parentChatRoomId: HexKey? = null,
+        val parentChatRoomIds: List<HexKey> = emptyList(),
         /**
          * Whether this device could open a subgroup here at all.
          *