title: How to use union types with interfaces in graphql
body: <p>Lets say I have a union type and an interface:</p>
  
  <pre><code>union WidgetType = A | B | C
  interface WidgetInterface {
      id: ID!
      name: String!
      type: WidgetType!
  }
  </code></pre>
  
  <p>How can I define a widget where the type is one of the unions?</p>
  
  <pre><code>type WidgetA implements WidgetInterface {
      id: ID!
      name: String!
      description: String
      type: ???
  }
  </code></pre>