Adds field selection RFC

[PascalSenn]

No description provided.

[smyrick]

From the meeting on May 2, @benjie has a comparable solution defined that builds on top of the schema coordinates proposal

In short, this pathing operator . is replaced with > and then selection is schema coordinates

https://github.com/graphql/graphql-wg/blob/main/rfcs/OperationExpressions.md

[benjie]

I've included some comments below, but I like the direction of the last solution.

To my mind the system knows if the input is a scalar or not. If it's a scalar, we expect a "single field" expression. If it's an input object, we expect a "multi field" expression (key1: expression1 key2: expression2); and for each of these expressions we apply the same heuristics: if it's a scalar, a single field expression, if an input object then a multi field expression wrapped in {} to avoid ambiguity.

My main concern is that the "expression" should use GraphQL syntax to explicitly select a single value. It's not terrible that this is implied via selection sets and the system knowing to traverse, but it would be better if this just "fell out" of the GraphQL syntax automatically, so I'm keen to see if exploring the flat chain syntax would get us any closer.

[benjie]

With operation expressions this would be:

Suggested change

	mediaById(id: ID! @is(field: "<Movie>.id | <Book>.id")): MovieOrBook @lookup
	mediaById(id: ID! @is(field: "Movie.id Book.id")): MovieOrBook @lookup

That would effectively compile out to: ... Movie { id } ... Book { id }.