Model relationship returning NULL

Question

I,m crafting out a modal where i can add/fetch data of post/posts in aws datastore. But for some reason i cant fetch the user data while fetching Post (returns null) even in appsync although i can fetch the remaining data (store, storeItems, & comments).

Here is my schema

type User @model @auth(rules: [{allow: private, operations: [create, read]}, {allow: owner}]) {
  id: ID!
  username: String
  avatar: String
  followers: String
  following: String
  Posts: [Post] @hasMany(indexName: "byUser", fields: ["id"])
}

type StoreItems @model @auth(rules: [{allow: private, operations: [create, read]}, {allow: owner}]) {
  id: ID!
  name: String
  quantity: Int
  price: Int
  storeID: ID! @index(name: "byStore")
  Store: Store @belongsTo
  photoArray: [String]
}

type Comment @model @auth(rules: [{allow: private, operations: [create, read]}, {allow: owner}]) {
  id: ID!
  comment: String
  likes: Int
  dislikes: Int
  replies: Int
  postID: ID! @index(name: "byPost")
  Post: Post @belongsTo
}

type Store @model @auth(rules: [{allow: private, operations: [create, read]}, {allow: owner}]) {
  id: ID!
  StoreItems: [StoreItems] @hasMany(indexName: "byStore", fields: ["id"])
  Post: Post @belongsTo
  storeName: String
}

type Post @model @auth(rules: [{allow: private, operations: [create, read]}, {allow: owner}]) {
  id: ID!
  title: String
  thumbnail: String
  media: [String]
  caption: String
  price: Int
  duration: Float
  views: Int
  likes: Int
  dislikes: Int
  Comments: [Comment] @hasMany(indexName: "byPost", fields: ["id"])
  Store: Store @hasOne
  userID: ID! @index(name: "byUser")
  User: User @belongsTo
}

my Post User resolver Request Mapping Template

#if( $ctx.source.deniedField )
  #return($util.toJson(null))
#end
#if( $util.isNull($ctx.source.userPostsId) )
  #return
#else
  #set( $GetRequest = {
  "version": "2018-05-29",
  "operation": "Query"
} )
  $util.qr($GetRequest.put("query", {
  "expression": "#partitionKey = :partitionValue",
  "expressionNames": {
      "#partitionKey": "id"
  },
  "expressionValues": {
      ":partitionValue": $util.parseJson($util.dynamodb.toDynamoDBJson($util.defaultIfNullOrBlank($ctx.source.userPostsId, "___xamznone____")))
  }
}))
  #if( !$util.isNullOrEmpty($ctx.stash.authFilter) )
    $util.qr($GetRequest.put("filter", $util.parseJson($util.transform.toDynamoDBFilterExpression($ctx.stash.authFilter))))
  #end
  $util.toJson($GetRequest)
#end

my Post User resolver response mapping template.

#if( $ctx.error )
$util.error($ctx.error.message, $ctx.error.type)
#else
  #if( !$ctx.result.items.isEmpty() && $ctx.result.scannedCount == 1 )
    $util.toJson($ctx.result.items[0])
  #else
    #if( $ctx.result.items.isEmpty() && $ctx.result.scannedCount == 1 )
$util.unauthorized()
    #end
    $util.toJson(null)
  #end
#end

What could be the problem?