Expose selected fields in resolver context

[0xSalman]

This fixes graph-gophers#17. Basically, it exposes the selected field tree in the context for the resolvers and downstream usage.

There are two PRs open in upstream that fix this issue but I think the solution in this PR is better and we can merge it in our branch to make use of it quickly. The PRs in upstream have been open for a while.

Does this break the existing api?

No

What if I do not want the selection tree? Does it have performance impact?

There should be very negligible performance impact. The selection tree is lazily added; meaning, it is added to the context only when you request it. The lazy evaluation happens via a closure function.

How can I use it?

In your resolver, you can do selected.GetFieldsFromContext(ctx) which returns []selected.SelectedField, error. For an example, take a look at example/social/social.go

Test Notes

• Run ./example/social/server/server.go
• Visit http://localhost:9011/ and run this query. Verify that some basic stuff about selection tree is logged in the console

query GetUser($userId: ID!, $page: Pagination) {
  user(id: $userId) {
    id
    name
    email
    phone
    friends(page: $page) {
      id
      name
    	email
    	phone
    }
  }
}

variables:

{
  "userId": "0x02",
  "page": {
    "first": 1,
    "last": 0
  }
}

TODO

• add unit tests

[0xSalman]

FYI, we are merging this PR to dt-master because we have a PR open against master in upstream and do not want these changes to appear there. I have made dt-master as the main branch for our fork to avoid conflicting with any open upstream PRs.

[Niennienzz]

Went through the code. Verified sample server with sample query. Also tried queries with more nested levels, it worked nicely.

[SteveGBanton]

Reviewed code, tested by integrating into helix repo, works.

[abourget]

I'd like to have that upstream :) Lots of forks around!