feat(query): add support for query by reference

[choyongjoon]

It's for the issue #103
I referenced schema.js
https://github.com/RisingStack/graffiti-mongoose/blob/master/src/schema/schema.js#L113

[tothandras]

Looks good! Can you add at least one test case?

[choyongjoon]

I've tried to make a test. But I got test errors.
Can you check it and show me the way?

choyongjoon@0b76253

[tothandras]

We should probably create end-to-end (e2e.spec.js) tests first.
I was playing with the model in #103.
1.

mutation AddForum {
  addForum(input: {title: "foo", clientMutationId: "1"}) {
    changedForumEdge {
      node {
        id
      }
    }
  }
}

mutation AddTopic {
  addTopic(input: {title: "bar", clientMutationId: "2", uuid: "uuid", forum: "id from previous response"}) {
    changedTopicEdge {
      node {
        id
      }
    }
  }
}

query TopicsByForum{
  topics(forum: "this with mongoose ObjectId, but it doesn't with GraphQL ID") {
    forum {
      id
      _id
      title
    }
  }
}

# error
"Cast to ObjectId failed for value \"Rm9ydW06NTcyNzlkM2RjNWQ2MDg3NTBkN2E5YzM4\" at path \"forum\""

The reason that it can't accept GraphQL ID is that here we only convert the id or _id fields from GraphQL to mongoose ID. We need to process all fields with something like:
selector = mapValues(selector, (field) => processId({id: field}));.

[developer-rakeshpaul]

@tothandras any luck on this PR?