Implement schema extend keyword

[emilebosch]

Approach to extending types such as https://graphql.github.io/graphql-spec/June2018/#sec-Type-Extensions

Todo

• Implement brute force extension of types in the GraphQL::Schema::BuildFromDefinition so it actually extends types and "works".
• Implement language printer vistors for printing back the document extend AST types to a string
• Find a way to declaratively build a GraphQL::Schema with type extensions and come up with a pretty way to do this. Extensions don't have a description so we fall flat making a new object.from GraphQL::BaseType. If the wasn't a problem how would it look like then? See below for a proposal. Also extensions aren't connected to the schema, since they sort of come after the fact?
• Make sure that we can convert from GraphQL::Schema back to GraphQL::Language::Document node. This mean tracking our extensions in some way or form so we can print it back. Check GraphQL::Language::DocumentFromSchemaDefinition and GraphQL::Schema::Printer
• Refactor the code to make more readable
• Tests maybe 🙈
• Get Roberts blessing

Questions

• How would we add type extensions to the API so that it gets generated properly?
• Should when asking for .fields on an ObjectType should it return all the fields?

Proposal for extensions

Extensions come after the fact,i.e. they should extend an existing type. However I have no clue how to do this elegantly. It's not like an interface where it gets implemented.

Sample: 1 Pure after the fact thing

# The original type
UserType = GraphQL::ObjectType.define do
  field :name, String
end

# How do we hook it up in the schema? Like orphans maybe?
UserTypeExtension = GraphQL::ObjectTypeExtension.define do
  extends UserType #I don't know the order either, so this might create a circular ref exception
  field :email, String
end

# Maybe this?
Schema.extensios << UserTypeExtension

Another way is to implement it like an interface, but that's not really what it is.

How is the JS world tackling this

tldr;; They have an extendSchema that extends the schema

• Type extension not in the AST graphql/graphql-js#1701
• buildASTSchema() should throw on encountering extend definition graphql/graphql-js#922
• How does type extension work? graphql/graphql-js#483