referenced_columns

# File activerecord/lib/active_record/relation/where_clause.rb, line 117
        def referenced_columns
          predicates.each_with_object({}) do |node, hash|
            attr = extract_attribute(node) || begin
              node.left if equality_node?(node) && node.left.is_a?(Arel::Predications)
            end

            hash[attr] = node if attr
          end
        end