merge_outer_joins

# File activerecord/lib/active_record/relation/merger.rb, line 136
        def merge_outer_joins
          return if other.left_outer_joins_values.empty?

          if other.model == relation.model
            relation.left_outer_joins_values |= other.left_outer_joins_values
          else
            associations, others = other.left_outer_joins_values.partition do |join|
              case join
              when Hash, Symbol, Array; true
              end
            end

            join_dependency = other.construct_join_dependency(
              associations, Arel::Nodes::OuterJoin
            )
            relation.left_outer_joins!(join_dependency, *others)
          end
        end