docs: def _joinOnAsPriv(self, model, onIndex, whatAs):
          """
          Private method for handling joins.
          """
          if self._join:
              raise Exception("Already joined with a table!")
  
          self._join = model
          self._joinedField = whatAs
          table = model.table
          self._query = self._query.eq_join(onIndex, r.table(table))
          return self