docs: def validate_on_submit(self):
          """ Extend validate on submit to allow validation with schema """
  
          # validate form
          valid = FlaskWtf.validate_on_submit(self)
  
          # return in case no schema or not submitted
          if not self._schema or not self.is_submitted():
              return valid
  
          # validate data with schema if got one and form was submitted
          data = dict()
          for field in self._fields:
              data[field] = self._fields[field].data
  
          result = self.schema.process(data, context=self._force_context)
          self.set_errors(result)
  
          # set filtered data back to form
          for field in data:
              self._fields[field].data = data[field]
  
          return valid and not bool(self.errors)