preloaders_for_hash

# File activerecord/lib/active_record/associations/preloader.rb, line 116
        def preloaders_for_hash(association, records, scope, polymorphic_parent)
          association.flat_map { |parent, child|
            grouped_records(parent, records, polymorphic_parent).flat_map do |reflection, reflection_records|
              loaders = preloaders_for_reflection(reflection, reflection_records, scope)
              recs = loaders.flat_map(&:preloaded_records).uniq
              child_polymorphic_parent = reflection && reflection.options[:polymorphic]
              loaders.concat Array.wrap(child).flat_map { |assoc|
                preloaders_on assoc, recs, scope, child_polymorphic_parent
              }
              loaders
            end
          }
        end