docs: def validate(self, value, model=None, context=None):
          """ Perform validation """
          from boiler.user.services import user_service
  
          self_id = None
          if model:
              if isinstance(model, dict):
                  self_id = model.get('id')
              else:
                  self_id = getattr(model, 'id')
  
          params = dict()
          params[self.property] = value
          found = user_service.first(**params)
          if not found or (model and self_id == found.id):
              return Error()
  
          return Error(self.error)