docs: def pk_field(self):
          '''
          Name of the primary key field as retrieved from rethinkdb table
          metadata, 'id' by default. Should not be overridden. Override
          `table_create` if you want to use a nonstandard field as the primary
          key.
          '''
          if not self._pk:
              try:
                  pk = self.rr.db('rethinkdb').table('table_config').filter({
                      'db': self.rr.dbname, 'name': self.table}).get_field(
                              'primary_key')[0].run()
                  self._pk = pk
              except Exception as e:
                  raise Exception(
                          'problem determining primary key for table %s.%s: %s',
                          self.rr.dbname, self.table, e)
          return self._pk