ids_reader

# File activerecord/lib/active_record/associations/collection_association.rb, line 45
      def ids_reader
        if loaded? || options[:finder_sql]
          load_target.map do |record|
            record.send(reflection.association_primary_key)
          end
        else
          column  = "#{reflection.quoted_table_name}.#{reflection.association_primary_key}"
          relation = scoped

          including = (relation.eager_load_values + relation.includes_values).uniq

          if including.any?
            join_dependency = ActiveRecord::Associations::JoinDependency.new(reflection.klass, including, [])
            relation = join_dependency.join_associations.inject(relation) do |r, association|
              association.join_relation(r)
            end
          end

          relation.pluck(column)
        end
      end