# File lib/arel/algebra/relations/relation.rb, line 60
    def primary_key
      connection_id = engine.connection.object_id
      if @@connection_tables_primary_keys[connection_id] && @@connection_tables_primary_keys[connection_id].has_key?(table.name)
        @@connection_tables_primary_keys[connection_id][table.name]
      else
        @@connection_tables_primary_keys[connection_id] ||= {}
        @@connection_tables_primary_keys[connection_id][table.name] = engine.connection.primary_key(table.name)
      end
    end