Union with types from different service implementing value type interface

[mpospelov]

• Package name: apollo-gateway occurs on master and latest versions

• Here a commit with failing test:

  • mpospelov/apollo-server@0a996c6
  • Given 2 schemas:

  type Query {
    content: [Content!]!
  }
  union Content = Video | Article
  type Article implements WebResource @key(fields: "id") {
    id: ID
    url: String
  }
  extend type Video @key(fields: "id") {
    id: ID @external
  }
  interface WebResource {
    url: String
  }

  interface WebResource {
    url: String
  }
  type Video implements WebResource @key(fields: "id") {
    id: ID
    url: String
  }

  • query via gateway with

  query {
    content {
      ... on WebResource {
        url
      }
    }
  }

  • the expected result is

  { 
    data:{
      content: [
        { url: 'https://foobar.com/articles/1' },
        { url: 'https://foobar.com/videos/1' },
      ]
    }
  }

  • the actual result is

  { 
    data:{
      content: [
        { url: 'https://foobar.com/articles/1' },
        { url: null },
      ]
    }
  }

  • Query plan:

   QueryPlan {
      Fetch(service: "contentService") {
        {
          content {
            __typename
            ... on WebResource {
              url
            }
          }
        }
      },
   }

  • it ignores querying the Video service for some reason, I believe that such cases should detect services which implements the interface to prevent this issue 🤔

• Runnable reproduction:

  1. clone my branch https://github.com/mpospelov/apollo-server/tree/union-with-interfaces-issue
  2. run jest -- packages/apollo-gateway/src/__tests__/integration/unions-with-value-interfaces.test.ts

Thank you for all the hard work! 🔥🚀