association_loader.rb

class AssociationLoader < GraphQL::Batch::Loader
  def initialize(association_name)
    @association_name = association_name
  end

  private

    def perform(records)
      ::ActiveRecord::Associations::Preloader.new.preload(records, @association_name)
      records.each { |record| fulfill(record, record.public_send(@association_name)) }
    end
end

record_loader.rb

class RecordLoader < 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