fix: disable GC on child collections created by includes system (#1430)

* test: add regression test for child collection GC data loss

Child collections created by the includes system (nested subqueries in
.select()) inherit the default gcTime of 5 minutes. When the only React
subscriber unmounts (e.g., virtual table scrolling), the collection gets
garbage collected and data is permanently lost.

This test subscribes to a child collection, unsubscribes (simulating
component unmount), advances past the GC timeout, and asserts the data
is still intact. It currently fails, proving the bug.

* fix: disable GC on child collections created by includes system

Add gcTime: 0 to createChildCollectionEntry so child collections are
not subject to time-based garbage collection.

Child collection lifecycle is already managed by flushIncludesState
Phase 5, which removes them from childRegistry when parent rows are
deleted. External GC is unnecessary and causes permanent data loss when
React subscribers unmount (e.g., virtual table scrolling, tab switching,
conditional rendering).

Fixes #1429

Author: blj

packages/db/src/query/live/collection-config-builder.ts

@@ -1515,6 +1515,7 @@ function createChildCollectionEntry(
       },
     },
     startSync: true,
+    gcTime: 0,
   })
 
   const entry: ChildCollectionEntry = {

packages/db/tests/query/includes.test.ts

@@ -1,4 +1,4 @@
-import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import {
   and,
   concat,
@@ -8,6 +8,7 @@ import {
   toArray,
 } from '../../src/query/index.js'
 import { createCollection } from '../../src/collection/index.js'
+import { CleanupQueue } from '../../src/collection/cleanup-queue.js'
 import { mockSyncCollectionOptions, stripVirtualProps } from '../utils.js'
 
 type Project = {
@@ -4012,4 +4013,52 @@ describe(`includes subqueries`, () => {
       })
     })
   })
+
+  describe(`child collection garbage collection`, () => {
+    beforeEach(() => {
+      vi.useFakeTimers()
+      CleanupQueue.resetInstance()
+    })
+
+    afterEach(() => {
+      vi.useRealTimers()
+      CleanupQueue.resetInstance()
+    })
+
+    it(`child collections should not be garbage collected when external subscribers unmount`, async () => {
+      const collection = buildIncludesQuery()
+      await collection.preload()
+
+      // Verify child data exists
+      const alpha = collection.get(1) as any
+      expect(childItems(alpha.issues)).toEqual([
+        { id: 10, title: `Bug in Alpha` },
+        { id: 11, title: `Feature for Alpha` },
+      ])
+
+      const beta = collection.get(2) as any
+      expect(childItems(beta.issues)).toEqual([
+        { id: 20, title: `Bug in Beta` },
+      ])
+
+      // Simulate what useLiveQuery does in React: subscribe to child collection,
+      // then unsubscribe when the component unmounts (e.g., virtual table scroll)
+      const childSub = alpha.issues.subscribeChanges(() => {})
+      childSub.unsubscribe()
+
+      // Advance well past the default gcTime (5 minutes = 300,000ms)
+      await vi.advanceTimersByTimeAsync(600_000)
+
+      // Child collection data should still be intact — the includes system
+      // owns these collections and manages their lifecycle via flushIncludesState.
+      // External GC must not destroy them.
+      expect(childItems(alpha.issues)).toEqual([
+        { id: 10, title: `Bug in Alpha` },
+        { id: 11, title: `Feature for Alpha` },
+      ])
+      expect(childItems(beta.issues)).toEqual([
+        { id: 20, title: `Bug in Beta` },
+      ])
+    })
+  })
 })