How to setup a GraphQL Relay API in Rails – Facebook api

I just want to leave here my findings for anyone who gets stuck with this in the future and wants to get pointed to a better direction.

First, the graphql-ruby gem includes everything that is needed to implement a Relay compatible GraphQL API. In includes everything that was before in the graphql-relay gem.

You need to provide 2 things in your Schema in order to make the Relay refetching feature to work well, an id_from_object method that converts an object in your domain, into a global id and also a object_from_id method that will decode the global id into an object in your application:

ApplicationSchema = GraphQL::Schema.define do
  /* Create IDs by joining the type name & ID, then base64-encoding it */
  id_from_object ->(object, type_definition, query_ctx) {
    GraphQL::Schema::UniqueWithinType.encode(type_definition.name, object.id)
  }

  object_from_id ->(id, query_ctx) {
    type_name, object_id = GraphQL::Schema::UniqueWithinType.decode(id)
    # Now, based on `type_name` and `id`
    # find an object in your application 
    # This will give the user access to all records in your db
    # so you might want to restrict this properly
    Object.const_get(type_name).find(object_id)
  }
end

Also, all your types should implement the NodeInterface provided by the ruby gem, and expose a global_id_field instead of an ID type:

PostType = GraphQL::ObjectType.define do
  name "Post"
  # Implements the "Node" interface for Relay
  interfaces [GraphQL::Relay::Node.interface]
  # exposes the  global id
  global_id_field :id
  field :name, types.String
end

This will allow Relay to refetch data like this:

query {
  node(id: "RmFjdGlvbjox") {
    id
    ... on Post {
      name
    }
  }
}

Relay also uses a babel-relay-plugin which requires a schema.json to be generated and available to the client, if you're building an isolated API with no view rendering, the way to go is to let the clients fetch the schema and not to do that work in the server, something like apollo-codegen can work. However, if you are building a rails app and need the schema in the same app, then you can run an instrospection query and save the result to a json file using a rake task:

Schema.execute GraphQL::Introspection::INTROSPECTION_QUERY

Lastly, you'll need to understand that Relay expresses one-to-many relationships with connections:

PostType = GraphQL::ObjectType.define do
  # default connection
  # obj.comments by default
  connection :comments, CommentType.connection_type

  # custom connection
  connection :featured_comments, CommentType.connection_type do
    resolve ->(post, args, ctx) {
      comments = post.comments.featured

      if args[:since]
        comments = comments.where("created_at >= ?", since)
      end

      comments
    }
  end
end

Connections support some arguments out of the box, you can use first, last, before and after in your connection queries:

query {
  posts(first: 5) {
    edges {
      node {
        name
      }
    }
  }
}

All of this is documented in the Relay documentation so make sure you read it as well as the graphql-ruby documentation.