code: def _add_model(self, model):
          
          name = model._name
          existing = self._models.get(name, None)
          if not existing:
              self._models[name] = model
          elif model.__name__ != existing.__name__ or model._creation_source != existing._creation_source:
              raise ImplementationError(
                  'A model with namespace "%s" and name "%s" is already defined '
                  'on this database' % (model.namespace, model.__name__))
          return self._models[name]
docs: Save this model as one existing on this database, to deny many models
          with same namespace and name.
          If the model already exists, check if it is the same. It can happen if the
          module is imported twice in different ways.
          If it's a new model or an existing and valid one, return the model in database: the
          one added or the existing one