fix(db): filter out-of-window SSE inserts in collection-subscriber

.changeset/fix-out-of-window-sse-filter.md

@@ -0,0 +1,5 @@
+---
+"@tanstack/db": patch
+---
+
+fix(db): filter SSE inserts outside the current window before they reach D2

packages/db/src/query/live/collection-subscriber.ts

@@ -248,15 +248,26 @@ export class CollectionSubscriber<
     // Use a holder to forward-reference subscription in the callback
     const subscriptionHolder: { current?: CollectionSubscription } = {}
 
+    const windowSize = limit + offset
     const sendChangesInRange = (
       changes: Iterable<ChangeMessage<any, string | number>>,
     ) => {
       const changesArray = Array.isArray(changes) ? changes : [...changes]
-
-      this.trackSentValues(changesArray, orderByInfo.comparator)
+      const windowBoundary = this.biggest
+      const relevant =
+        windowBoundary === undefined || windowSize === Infinity
+          ? changesArray
+          : changesArray.filter((change) => {
+              if (change.type !== `insert`) return true
+              if (this.sentToD2Keys.has(change.key)) return true
+              if (this.sentToD2Keys.size < windowSize) return true
+              return orderByInfo.comparator(windowBoundary, change.value) >= 0
+            })
+
+      this.trackSentValues(relevant, orderByInfo.comparator)
 
       // Split live updates into a delete of the old value and an insert of the new value
-      const splittedChanges = splitUpdates(changesArray)
+      const splittedChanges = splitUpdates(relevant)
       this.sendChangesToPipelineWithTracking(
         splittedChanges,
         subscriptionHolder.current!,

packages/db/tests/collection-subscriber-out-of-window-sse.test.ts

@@ -0,0 +1,87 @@
+import { describe, expect, it } from 'vitest'
+import { createCollection } from '../src/collection/index.js'
+import { BTreeIndex } from '../src/indexes/btree-index.js'
+import { createLiveQueryCollection } from '../src/query/index.js'
+import { mockSyncCollectionOptions } from './utils.js'
+
+type TestItem = {
+  id: string
+  value: number
+}
+
+describe(`CollectionSubscriber out-of-window SSE filter`, () => {
+  it(`should not promote an out-of-window SSE insert when an in-window item is deleted`, async () => {
+    const initialData: Array<TestItem> = [
+      { id: `1`, value: 100 },
+      { id: `2`, value: 90 },
+      { id: `3`, value: 80 },
+      { id: `4`, value: 70 },
+    ]
+
+    const sourceCollection = createCollection(
+      mockSyncCollectionOptions({
+        id: `sse-window-filter`,
+        getKey: (item: TestItem) => item.id,
+        initialData,
+        autoIndex: `eager`,
+        defaultIndexType: BTreeIndex,
+      }),
+    )
+
+    await sourceCollection.preload()
+
+    const liveQueryCollection = createLiveQueryCollection((q) =>
+      q
+        .from({ items: sourceCollection })
+        .orderBy(({ items }) => items.value, `desc`)
+        .limit(3)
+        .select(({ items }) => ({
+          id: items.id,
+          value: items.value,
+        })),
+    )
+
+    await liveQueryCollection.preload()
+
+    const initialResults = Array.from(liveQueryCollection.values())
+    expect(initialResults).toHaveLength(3)
+    expect(initialResults.map((r) => r.id)).toEqual([`1`, `2`, `3`])
+
+    // SSE delivers an insert for an item that sorts BELOW the current window
+    // (value 10 is lower than all top-3 values: 100, 90, 80)
+    sourceCollection.utils.begin()
+    sourceCollection.utils.write({
+      type: `insert`,
+      value: { id: `out-of-window`, value: 10 },
+    })
+    sourceCollection.utils.commit()
+
+    // Window should be unchanged
+    let results = Array.from(liveQueryCollection.values())
+    expect(results).toHaveLength(3)
+    expect(results.map((r) => r.id)).toEqual([`1`, `2`, `3`])
+
+    // Now delete one of the top-3 items
+    sourceCollection.utils.begin()
+    sourceCollection.utils.write({
+      type: `delete`,
+      value: { id: `2`, value: 90 },
+    })
+    sourceCollection.utils.commit()
+
+    // Wait for loadNextItems to fetch the replacement
+    await new Promise((resolve) => setTimeout(resolve, 50))
+
+    results = Array.from(liveQueryCollection.values())
+    expect(results).toHaveLength(3)
+
+    // The replacement should be item 4 (value 70) — the next item from the
+    // BTree — NOT the out-of-window SSE insert (value 10).
+    expect(
+      results.map((r) => r.id),
+      `Expected item 4 (value 70) to replace deleted item 2, ` +
+        `not the out-of-window SSE insert (value 10). ` +
+        `Got: ${JSON.stringify(results.map((r) => ({ id: r.id, value: r.value })))}`,
+    ).toEqual([`1`, `3`, `4`])
+  })
+})