build_join_buckets

# File activerecord/lib/active_record/relation/query_methods.rb, line 1820
      def build_join_buckets
        buckets = Hash.new { |h, k| h[k] = [] }

        unless left_outer_joins_values.empty?
          stashed_left_joins = []
          left_joins = select_named_joins(left_outer_joins_values, stashed_left_joins) do |left_join|
            if left_join.is_a?(CTEJoin)
              buckets[:join_node] << build_with_join_node(left_join.name, Arel::Nodes::OuterJoin)
            else
              raise ArgumentError, "only Hash, Symbol and Array are allowed"
            end
          end

          if joins_values.empty?
            buckets[:named_join] = left_joins
            buckets[:stashed_join] = stashed_left_joins
            return buckets, Arel::Nodes::OuterJoin
          else
            stashed_left_joins.unshift construct_join_dependency(left_joins, Arel::Nodes::OuterJoin)
          end
        end

        joins = joins_values.dup
        if joins.last.is_a?(ActiveRecord::Associations::JoinDependency)
          stashed_eager_load = joins.pop if joins.last.base_klass == model
        end

        joins.each_with_index do |join, i|
          joins[i] = Arel::Nodes::StringJoin.new(Arel.sql(join.strip)) if join.is_a?(String)
        end

        while joins.first.is_a?(Arel::Nodes::Join)
          join_node = joins.shift
          if !join_node.is_a?(Arel::Nodes::LeadingJoin) && (stashed_eager_load || stashed_left_joins)
            buckets[:join_node] << join_node
          else
            buckets[:leading_join] << join_node
          end
        end

        buckets[:named_join] = select_named_joins(joins, buckets[:stashed_join]) do |join|
          if join.is_a?(Arel::Nodes::Join)
            buckets[:join_node] << join
          elsif join.is_a?(CTEJoin)
            buckets[:join_node] << build_with_join_node(join.name)
          else
            raise "unknown class: %s" % join.class.name
          end
        end

        buckets[:stashed_join].concat stashed_left_joins if stashed_left_joins
        buckets[:stashed_join] << stashed_eager_load if stashed_eager_load

        return buckets, Arel::Nodes::InnerJoin
      end