Saving nested models in Android may raise GraphQL error for iOS devices that subscribe to the same models

[HuiSF]

This issue is caused by that platform libraries generating incompatible selection set making mutation requests.
In most cases, especially when nested models are involved, iOS platform cannot receive subscription updates from mutations made in other platforms, such as amplify-js (Amplify Studio), amplify-android.

Describe the bug
Saving nested models in Android may raise GraphQL error for iOS devices that subscribe to the same models. It happens when saving nested models in Android, and receiving subscription message in iOS.

Expand for details

This is due to Android doesn't populate nested models by default, when saving object, service side GraphQL validation may fail due to missing fields in the request sent from Android. The error may be emitted to all model subscriptions in different devices.

Two possible causes:

1. In Android, nested models (has-one, belongs-to) are not populated with data
2. When saving an object with creating association with other models, complete objects of other models see to be required, but no associating only by id field

Android sent mutation that caused the error

{
  "query": "mutation CreateComment($input: CreateCommentInput!) {\n  createComment(input: $input) {\n    _deleted\n    _lastChangedAt\n    _version\n    content\n    id\n    post {\n      id\n    }\n  }\n}\n",
  "variables": {
    "input": {
      "id": "2a7693b3-0591-4941-901e-fb355c962f72",
      "postID": "a4930282-77ea-468d-88e2-c042a8e575a5",
      "content": "tetetet"
    }
  }
}

To Reproduce
Steps to reproduce the behavior:

1. Run amplify_datastore with API sync in 1 Android emulator and 1 iOS simulator
2. Save a new "Comment" in Android emulator
3. Observe iOS simulator logs
4. See error

2021-06-22 12:59:28.065189-0700 Runner[5863:11863431] [StarscreamAdapter] websocketDidReceiveMessage: - {"id":"13C8F47B-5693-4925-9818-9A404031CC15","type":"data","payload":{"data":{"onCreateComment":{"id":"8360f342-5515-4a49-98e1-906211efd408","content":"Test","post":null,"__typename":"Comment","_version":1,"_deleted":null,"_lastChangedAt":1624391967947}},"errors":[{"message":"Cannot return null for non-nullable type: 'Int' within parent 'Post' (/onCreateComment/post/rating)","path":["onCreateComment","post","rating"]},{"message":"Cannot return null for non-nullable type: 'String' within parent 'Post' (/onCreateComment/post/title)","path":["onCreateComment","post","title"]},{"message":"Cannot return null for non-nullable type: 'Person' within parent 'Post' (/onCreateComment/post/author)","path":["onCreateComment","post","author"]},{"message":"Cannot return null for non-nullable type: 'Int' within parent 'Post' (/onCreateComment/post/_version)","path":["onCreateComment","post","_version"]},{"message":"Cannot return null for non-nullable type: 'AWSTimestamp' within parent 'Post' (/onCreateComment/post/_lastChangedAt)","path":["onCreateComment","post","_lastChangedAt"]}]}}

Expected behavior
iOS should receive correct subscription message.

Platform
Amplify Flutter current supports iOS and Android. This issue is reproducible in (check all that apply):
[ x ] Android
[ x ] iOS

[HuiSF]

Discovered that amplify-android has a bug that the SelectionSet class never generates GraphQL document correctly, as the getModelFields for Flutter uses ModelSchemaRegistry as singleton but it actually is not a singleton so it cannot retrieve related model schema.

[HuiSF]

Selection set incompatibility.

	amplify-js	amplify-ios	amplify-android	amplify-flutter	amplify-flutter
				iOS platform	Android platform
Create mutation selection set	createPost(input: $input) { id title rating createdAt updatedAt _version _lastChangedAt _deleted blog { id _deleted } }	createPost(input: $input) { id createdAt rating title updatedAt blog { id createdAt name updatedAt __typename _version _deleted _lastChangedAt } __typename _version _deleted _lastChangedAt }	createPost(input: $input) { _deleted _lastChangedAt _version blog { id } comments { items { id } nextToken startedAt } createdAt id rating title updatedAt }	Same as amplify-ios	Same as amplify-android
Update mutation selection set	updatePost(input: $input) { id title rating createdAt updatedAt _version _lastChangedAt _deleted blog { id _deleted } }	updatePost(input: $input) { id createdAt rating title updatedAt blog { id createdAt name updatedAt __typename _version _deleted _lastChangedAt } __typename _version _deleted _lastChangedAt }	updatePost(input: $input) { _deleted _lastChangedAt _version blog { id } comments { items { id } nextToken startedAt } createdAt id rating title updatedAt }	Same as amplify-ios	Same as amplify-android
Delete mutation selection set	deletePost(input: $input) { id title rating createdAt updatedAt _version _lastChangedAt _deleted blog { id _deleted } }	deletePost(input: $input) { id createdAt rating title updatedAt blog { id createdAt name updatedAt __typename _version _deleted _lastChangedAt } __typename _version _deleted _lastChangedAt }	deletePost(input: $input) { _deleted _lastChangedAt _version blog { id } comments { items { id } nextToken startedAt } createdAt id rating title updatedAt }	Same as amplify-ios	Same as amplify-android
onCreate subscription selection set	onCreatePost { id title rating createdAt updatedAt _version _lastChangedAt _deleted blog { id _deleted } }	onCreatePost { id createdAt rating title updatedAt blog { id createdAt name updatedAt __typename _version _deleted _lastChangedAt } __typename _version _deleted _lastChangedAt }	onCreatePost { _deleted _lastChangedAt _version blog { id } comments { items { id } nextToken startedAt } createdAt id rating title updatedAt }	Same as amplify-ios	Same as amplify-android
onUpdate subscription selection set	onUpdatePost { id title rating createdAt updatedAt _version _lastChangedAt _deleted blog { id _deleted } }	onUpdatePost { id createdAt rating title updatedAt blog { id createdAt name updatedAt __typename _version _deleted _lastChangedAt } __typename _version _deleted _lastChangedAt }	onUpdatePost { _deleted _lastChangedAt _version blog { id } comments { items { id } nextToken startedAt } createdAt id rating title updatedAt }	Same as amplify-ios	Same as amplify-android
onDelete subscription selection set	onDeletePost { id title rating createdAt updatedAt _version _lastChangedAt _deleted blog { id _deleted } }	onDeletePost { id createdAt rating title updatedAt blog { id createdAt name updatedAt __typename _version _deleted _lastChangedAt } __typename _version _deleted _lastChangedAt }	onDeletePost { _deleted _lastChangedAt _version blog { id } comments { items { id } nextToken startedAt } createdAt id rating title updatedAt }	Same as amplify-ios	Same as amplify-android

[yazoonic]

any update on this? I have the same problem but only when I add record from ios , I got error in Android. but when I add record from android, ios is fine and update record ..

I am SMH here, because in my dev environment it works with no issues.

[yazoonic]

the issue was that my dev database was recreated so fields like created at and updated at was shipped in models being generated ... now issue fixed. Thank you and ignore my comment above.