fix: honor deletions from AppSync when used by Flutter (#1004)

When the DataStore is used from Flutter, Java-language model classes
don't exist.

The Merger component is responsible for applying remote mutations onto
the local store. It contained an ifPresent() check, which checked for
the presence of a model by its Java class. This method would fail to
work as expected when the SerializedModel envelope type was used.

ifPresent() has been updated to query the local store by model name,
instead of using the Java class.

Resolves: #994

Author: jamesonwilliams

aws-datastore/src/main/java/com/amplifyframework/datastore/syncengine/Merger.java

@@ -27,6 +27,7 @@
 import com.amplifyframework.datastore.DataStoreChannelEventName;
 import com.amplifyframework.datastore.appsync.ModelMetadata;
 import com.amplifyframework.datastore.appsync.ModelWithMetadata;
+import com.amplifyframework.datastore.appsync.SerializedModel;
 import com.amplifyframework.datastore.storage.LocalStorageAdapter;
 import com.amplifyframework.datastore.storage.StorageItemChange;
 import com.amplifyframework.hub.HubChannel;
@@ -132,7 +133,7 @@ private <T extends Model> Completable delete(T model, Consumer<StorageItemChange
             // First, check if the thing exists.
             // If we don't, we'll get an exception saying basically,
             // "failed to delete a non-existing thing."
-            ifPresent(model.getClass(), model.getId(),
+            ifPresent(model,
                 () -> localStorageAdapter.delete(
                     model,
                     StorageItemChange.Initiator.SYNC_ENGINE,
@@ -165,17 +166,20 @@ private <T extends Model> Completable save(T model, Consumer<StorageItemChange<T
     }
 
     /**
-     * If the DataStore contains an item of the given class and with the given ID,
-     * then perform an action. Otherwise, perform some other action.
-     * @param clazz Search for this class in the DataStore
-     * @param modelId Search for an item with this ID in the DataStore
+     * If the DataStore contains a model instance, then perform an action.
+     * Otherwise, perform some other action.
+     * @param model A model that might exist in local storage
      * @param onPresent If there is a match, perform this action
      * @param onNotPresent If there is NOT a match, perform this action as a fallback
-     * @param <T> The type of item being searched
      */
-    private <T extends Model> void ifPresent(
-            Class<T> clazz, String modelId, Action onPresent, Action onNotPresent) {
-        localStorageAdapter.query(clazz, Where.id(modelId), iterator -> {
+    private void ifPresent(Model model, Action onPresent, Action onNotPresent) {
+        final String modelName;
+        if (model instanceof SerializedModel) {
+            modelName = ((SerializedModel) model).getModelName();
+        } else {
+            modelName = model.getClass().getSimpleName();
+        }
+        localStorageAdapter.query(modelName, Where.id(model.getId()), iterator -> {
             if (iterator.hasNext()) {
                 onPresent.call();
             } else {