title: Subtypes in GraphQL mutation
body: <p>I am attempting to specify a subtype in a graphql mutation
  Using AWS Amplify and AWS AppSync.</p>
  
  <pre><code>mutation CreatePhoneType {
    createPhoneType(input: {
      desc: "Mobile Phone"
    }) {
      id
      desc
    }
  }
  
  mutation CreatePhone {
    createPhone(input: {
      number: "+91 704-011-2342",
      phonetype: { id: "f6b3c538-378a-48d1-a264-96dfef4472a4" }    
    }) {
      id
      number,
      phonetype {
        id
      }
    }
  }
  </code></pre>
  
  <p>Here is the schema definition:</p>
  
  <pre><code>type Phone @model {
    id: ID!
    number: AWSPhone!
    phonetype: PhoneType
    createdAt: AWSDateTime
    updatedAt: AWSDateTime
  }
  
  type PhoneType @model {
    id: ID!
    desc: String!
    createdAt: AWSDateTime
    updatedAt: AWSDateTime
  }
  </code></pre>
  
  <p>I'm attempting to insert a new phone value. The Phone type has a subtype of PhoneType. I've created the PhoneType successfully and would like to reference the existing phone type as part of a mutation to create a Phone value. I've tried specifying the ID! but that does not work. How can you specify an existing subtype when creating a new value in a GraphQL mutation?
  I've checked the GraphQL docs but there is not a lot of information.</p>