code: def _validate_table(self, term):
          
          base_tables = self._from + [self._update_table]
  
          for field in term.fields():
              table_in_base_tables = field.table in base_tables
              table_in_joins = field.table in [join.item for join in self._joins]
              if field.table is not None \
                    and not table_in_base_tables \
                    and not table_in_joins \
                    and field.table != self._update_table:
                  return False
          return True
docs: Returns False if the term references a table not already part of the
          FROM clause or JOINS and True otherwise.