babel plugin - support introspection fields

[josephsavona]

Supports compiling introspection queries/fields:

• query { __schema }
• query { __type(name: "Name") }
• __typename on any field.

[josephsavona]

addresses #268 #162

[josephsavona]

cc @devknoll @drabinowitz

[devknoll]

👍

Daydreaming a little bit, it might be nice if the sort of thing in #268 could be baked into Relay (or a Relay add-on) in some capacity in a way that prevents developer errors. I'm imagining something like:

post {
  fragment on Post {
    ${Video.getFragment('video')},
    ${Picture.getFragment('picture')}
  }
}

(notice that there's no explicit __typename query)

Instead, a developer would just do something like:

const Component = this.props.relay.matchFragmentType(this.props.post, {
  'Video': () => Video,
  'Picture': () => Picture,
});

Now the babel plugin could find any calls to matchFragmentType and automatically insert __typename as appropriate. There would be an invariant thrown if a key passed in does not match one of the possible types, and a warning if there is some type that is not handled (and perhaps a default key or parameter added to handle/silence that case).

Thoughts?

[josephsavona]

@devknoll interesting. can you file an issue to discuss the idea?