chore: add changeset for object field update rollback fix

Author: samwillis

.changeset/fix-object-field-update-rollback.md

@@ -0,0 +1,8 @@
+---
+"@tanstack/query-db-collection": patch
+"@tanstack/db": patch
+---
+
+fix(query-db-collection): use deep equality for object field comparison in query observer
+
+Fixed an issue where updating object fields (non-primitives) with `refetch: false` in `onUpdate` handlers would cause the value to rollback to the previous state every other update. The query observer was using shallow equality (`===`) to compare items, which compares object properties by reference rather than by value. This caused the observer to incorrectly detect differences and write stale data back to syncedData. Now uses `deepEquals` for proper value comparison.

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

@@ -800,27 +800,14 @@ export function queryCollectionOptions(
 
           begin()
 
-          // Helper function for deep equality check of objects
-          const itemsEqual = (
-            obj1: Record<string, any>,
-            obj2: Record<string, any>
-          ): boolean => {
-            return deepEquals(obj1, obj2)
-          }
-
           currentSyncedItems.forEach((oldItem, key) => {
             const newItem = newItemsMap.get(key)
             if (!newItem) {
               const needToRemove = removeRow(key, hashedQueryKey) // returns true if the row is no longer referenced by any queries
               if (needToRemove) {
                 write({ type: `delete`, value: oldItem })
               }
-            } else if (
-              !itemsEqual(
-                oldItem as Record<string, any>,
-                newItem as Record<string, any>
-              )
-            ) {
+            } else if (!deepEquals(oldItem, newItem)) {
               // Only update if there are actual differences in the properties
               write({ type: `update`, value: newItem })
             }