title: Nested mutation GraphQL
body: <p>I'm using AWS Appsync and Amplify.
  A snippet of my GraphQL schema look like this:</p>
  
  <pre><code>type Recipe
  @model
  @auth(rules: [{allow: owner}])
  {
   id: ID!
   title: String!
   key: String!
   courses: [Course!]!
  }
  
  type Course
  @model
  @auth(rules: [{allow: owner}])
  {
    id: ID!
    name: String!
  }
  </code></pre>
  
  <p>On amplify push it creates the DynamoDB tables <code>Recipe</code> and <code>Course</code></p>
  
  <p>After reading many tutorials I still don't get it how to add a recipe in GraphiQL.</p>
  
  <p>How can i insert a new Recipe that has a reference to a course and avoid duplicates in the Courses table?</p>