fix(datastore): Setting nil values for empty associated models (#2358)

* fix(datastore): Setting nil values for empty associated models

* address PR comments

* test clean up

Author: lawmicha

AmplifyPlugins/Core/AWSPluginsCore/Model/Support/Model+GraphQL.swift

@@ -41,12 +41,28 @@ extension Model {
             let name = modelField.name
 
             guard let value = modelFieldValue else {
-                // don't invalidate fields of type .model
-                // as we'll take care of this later on (see line 61)
+                // Special case for associated models when the value is `nil`, by setting all of the associated
+                // model's primary key fields (targetNames) to `nil`.
                 if case .model = modelField.type {
-                    continue
+                    let fieldNames = getFieldNameForAssociatedModels(modelField: modelField)
+                    for fieldName in fieldNames {
+                        // Only set to `nil` if it has not been set already. For hasOne relationships, where the
+                        // target name of the associated model is explicitly on this model as a field property, we
+                        // cannot guarantee which field is processed first, thus if there is a value for the explicit
+                        // field and was already set, don't overwrite it.
+                        if input[fieldName] == nil {
+                            // Always setting the value to `nil` is not necessary for create mutations, since leaving it
+                            // out will also reflect that there's no associated model. However, we always set it to `nil`
+                            // to account for the update mutation use cases where the caller may be de-associating the
+                            // model from the associated model, which is why the `nil` is required in input variables
+                            // to persist the removal the association.
+                            input.updateValue(nil, forKey: fieldName)
+                        }
+                    }
+                } else {
+                    input.updateValue(nil, forKey: name)
                 }
-                input.updateValue(nil, forKey: name)
+                
                 continue
             }
 

AmplifyPlugins/DataStore/AWSDataStoreCategoryPluginIntegrationTests/TransformerV2/DataStoreConnectionOptionalAssociations.swift

@@ -199,19 +199,38 @@ class DataStoreConnectionOptionalAssociations: SyncEngineIntegrationV2TestBase {
         XCTAssertEqual(queriedPost.blog?.id, blog.id)
 
         queriedComment.post = nil
-        queriedPost.blog = nil
-        guard savePost(queriedPost) != nil,
-              saveComment(queriedComment) != nil else {
-            XCTFail("Failed to update comment and post")
+        // A mock GraphQL request is created to assert that the request variables contains the "postId"
+        // with the value `nil` which is sent to the API to persist the removal of the association.
+        let request = GraphQLRequest<Comment8>.createMutation(of: queriedComment, version: 1)
+        guard let variables = request.variables,
+              let input = variables["input"] as? [String: Any?],
+              let postValue = input["postId"],
+              postValue == nil else {
+            XCTFail("Failed to retrieve input object from GraphQL variables")
             return
         }
-        guard let queriedCommentWithoutPost = queryComment(id: comment.id),
-              let queriedPostWithoutBlog = queryPost(id: post.id) else {
-            XCTFail("Failed to query comment and post")
+
+        guard saveComment(queriedComment) != nil else {
+            XCTFail("Failed to update comment")
+            return
+        }
+        guard let queriedCommentWithoutPost = queryComment(id: comment.id) else {
+            XCTFail("Failed to query comment without post")
             return
         }
 
         XCTAssertNil(queriedCommentWithoutPost.post)
+
+        queriedPost.blog = nil
+        guard savePost(queriedPost) != nil else {
+            XCTFail("Failed to update comment and post")
+            return
+        }
+
+        guard let queriedPostWithoutBlog = queryPost(id: post.id) else {
+            XCTFail("Failed to query post")
+            return
+        }
         XCTAssertNil(queriedPostWithoutBlog.blog)
     }