Use graphql-batch with graphql-ruby to resolve n + 1 issue

item_type.rb

module Types
  class ItemType < GraphQL::Schema::Object
    field :id, ID, null: false
    field :title, String, null: false
    field :description, String, null: true
    field :image_url, String, null: true
    # (graphql-ruby 1.9.12)
    # instead of using `resolve` inside block of `field` like this:
    #
    # field :user, Types::UserType, null: false do
    #   resolve ->(item, args, ctx) do
    #     Loaders::PrimaryKeyLoader.for(User).load item.user_id
    #   end
    # end
    #
    # we should use a pair of key-value like below:
    field :user, Types::UserType, null: false, resolve: ->(item, args, ctx) do
      Loaders::PrimaryKeyLoader.for(User).load item.user_id
    end
  end
end

primary_key_loader.rb

module Loaders
  class PrimaryKeyLoader < GraphQL::Batch::Loader
    def initialize model
      @model = model
    end

    def perform ids
      @model.where(id: ids).each{|record| fulfill record.id, record}
      ids.each{|id| fulfill(id, nil) unless fulfilled?(id)}
    end
  end
end

user_type.rb

module Types
  class UserType < GraphQL::Schema::Object
    field :id, ID, null: false
    field :email, String, null: false
    field :full_name, String, null: false

    def full_name
      [object.first_name, object.last_name].compact.join " "
    end
  end
end