Many-To-Many Connections have incorrect generated mutations for join model

[ghost]

Describe the bug
Take example schema from page https://aws-amplify.github.io/docs/cli-toolchain/graphql#connection
Many-To-Many Connections and run the related example mutations.

The CreateLinks mutation fails.

This prevents filling up the join model table with data, thus making many to many connections not usable.

type Post @model {
  id: ID!
  title: String!
  editors: [PostEditor] @connection(keyName: "byPost", fields: ["id"])
}

# Create a join model and disable queries as you don't need them
# and can query through Post.editors and User.posts
type PostEditor
  @model(queries: null)
  @key(name: "byPost", fields: ["postID", "editorID"])
  @key(name: "byEditor", fields: ["editorID", "postID"]) {
  id: ID!
  postID: ID!
  editorID: ID!
  post: Post! @connection(fields: ["postID"])
  editor: User! @connection(fields: ["editorID"])
}

type User @model {
  id: ID!
  username: String!
  posts: [PostEditor] @connection(keyName: "byEditor", fields: ["id"])
}

Execution of CreateLinks mutation fails:

mutation CreateData {
    p1: createPost(input: { id: "P1", title: "Post 1" }) {
        id
    }
    p2: createPost(input: { id: "P2", title: "Post 2" }) {
        id
    }
    u1: createUser(input: { id: "U1", username: "user1" }) {
        id
    }   
    u2: createUser(input: { id: "U2", username: "user2" }) {
        id
    }
}

mutation CreateLinks {
    p1u1: createPostEditor(input: { id: "P1U1", postID: "P1", editorID: "U1" }) {
        id
    }   
    p1u2: createPostEditor(input: { id: "P1U2", postID: "P1", editorID: "U2" }) {
        id
    }
    p2u1: createPostEditor(input: { id: "P2U1", postID: "P2", editorID: "U1" }) {
        id
    }
}

Error message:

Validation error of type WrongType: **argument 'input'** with value 
'ObjectValue{
objectFields=[ObjectField{name='id', value=StringValue{value='P1U1'}}, ObjectField{name='postID', value=StringValue{value='P1'}}, ObjectField{name='editorID', value=StringValue{value='U1'}}]}' **is missing required fields '[post]' @ 'createPostEditor'**

Amplify CLI Version
4.13.3

To Reproduce
Setup example schema and run example mutations

Expected behavior
Mutation CreateLinks is executed successfully.

Screenshots
none

Desktop (please complete the following information):

• OS: Mac
• Node Version: v12.16.0

Additional context
The generated schema type CreatePostEditorInput contains post: PostInput! field which is probably the root cause of the issue.

input CreatePostEditorInput {
  id: ID
  postID: ID!
  editorID: ID!
  post: PostInput!
  _version: Int
}

[SwaySway]

Hello @silvesteradamik the bug here is that the input type is requiring the post in CreatePostEditorInput type. A potential workaround here would be to make the connected fields nullable. This would not make a requirement to add the post type in the mutation.

[ghost]

Making the connected fields nullable solved the problem with filling the join connection table.

However, there is now visible different issue:
I can get correct result when listing Users - I can see posts related to them.
I get empty usesrs in listing Posts - I can't see users related to posts.

When fixing the issue, please check that the relations between the Editor and Post tables work bidirectional.

[attilah]

This is only happening when conflict resolution is enabled, without that Transformer is behaving correctly.

@silvesteradamik Please confirm that you've enabled conflict resolution for your API.

[amuresia]

Hi @attilah I can see the same error and I can confirm that I don't have conflict resolution enabled.
Schema:

type Blog @model {
  id: ID!
  name: String!
  posts: [Post] @connection(keyName: "byBlog", fields: ["id"])
}

type Post @model @key(name: "byBlog", fields: ["blogID"]) {
  id: ID!
  title: String!
  blogID: ID!
  blog: Blog @connection(fields: ["blogID"])
  comments: [Comment] @connection(keyName: "byPost", fields: ["id"])
}

type Comment @model @key(name: "byPost", fields: ["postID", "content"]) {
  id: ID!
  postID: ID!
  post: Post @connection(fields: ["postID"])
  content: String!
}

transform.conf.json

{
    "Version": 5,
    "ElasticsearchWarning": true
}

Error

Completed with error: GraphQL service returned a successful response containing errors: [Amplify.GraphQLError(message: "Variable \'input\' has coerced Null value for NonNull type \'ID!\'", locations: Optional([Amplify.GraphQLError.Location(line: 1, column: 21)]), path: nil, extensions: nil)]

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