preloaders_for_reflection

# File activerecord/lib/active_record/associations/preloader/branch.rb, line 85
        def preloaders_for_reflection(reflection, reflection_records)
          reflection_records.group_by do |record|
            klass = record.association(association).klass

            if reflection.scope && reflection.scope.arity != 0
              # For instance dependent scopes, the scope is potentially
              # different for each record. To allow this we'll group each
              # object separately into its own preloader
              reflection_scope = reflection.join_scopes(klass.arel_table, klass.predicate_builder, klass, record).inject(&:merge!)
            end

            [klass, reflection_scope]
          end.map do |(rhs_klass, reflection_scope), rs|
            preloader_for(reflection).new(rhs_klass, rs, reflection, scope, reflection_scope, associate_by_default)
          end
        end