listing_type.rb

ListingType = GraphQL::ObjectType.define do
  name "Listing"
  description "Book listing by user"
  global_id_field :id
  field :id, !types.ID
  field :user, !UserType, "if of user who owns book"
  field :price, !types.Float
end

user_type.rb

UserType = GraphQL::ObjectType.define do
  name "User"
  description "Application user"
  interfaces [NodeIdentification.interface]

  field :id, !types.ID
  global_id_field :id
  field :username, !types.String, "The name the person chose upon registration"
  connection :listings, ListingType.connection_type, "Listings for user"
end