docs: def _from_rest_reject_update(model):
      """ Reject any field updates not allowed on POST
  
      This is done on fields with `reject_update=True`.
      """
  
      dirty = model.dirty_fields
      fields = model.get_fields_by_prop('reject_update', True)
      reject = []
  
      for field in fields:
          if field in dirty:
              reject.append(field)
  
      if reject:
          mod_fail('These fields cannot be updated: %s' % ', '.join(reject))