title: GraphQL resolve GraphQLObjectType
body: <p>I'm using <a href="https://github.com/graphql/express-graphql" rel="nofollow noreferrer" title="express-graphql">express-graphql</a> along with the following query:</p>
  
  <pre><code>invitations {
      sent 
      received
  }
  </code></pre>
  
  <p>The schema definition (simplified) is as follows:</p>
  
  <pre><code>const InvitationType = new GraphQLObjectType({
      name: 'InvitationType',
      description: 'Friends invitations',
      fields: {
          sent: {
              type: new GraphQLList(GraphQLString),
              description: 'Invitations sent to friends.',
              resolve() {
               return ['sentA'];
              }
          },
          received: {
              type: new GraphQLList(GraphQLString),
              description: 'Invitations received from friends.',
              resolve() {
               return ['receivedA', 'receivedB'];
              }
          }
      }
  });
  
  // Root schema
  const schema = new GraphQLSchema({
      query: new GraphQLObjectType({
        name: 'RootQueryType',
        fields: {
            invitations: {
                type: InvitationType // no resolve() method here.
            }
        }
      })
  });
  </code></pre>
  
  <p>However the <code>resolve</code> methods are never called for <code>sent</code> and <code>received</code> fields. The query above returns:</p>
  
  <pre><code>{data: {invitations: {sent: null, received: null}}}
  </code></pre>
  
  <p>Is there any way to resolve the nested fields (<code>sent</code> and <code>received</code>) without defining a <code>resolve()</code> method on parent (<code>invitations</code>) field?</p>