Check if a relation with the current related_name doesn't already exists
        for the related model

def _assert_relation_does_not_exists(self):
        """
        Check if a relation with the current related_name doesn't already exists
        for the related model
        """
        relations = self.database._relations[self.related_to]
        existing = [r for r in relations if r[2] == self.related_name]
        if existing:
            error = ("The related name defined for the field '%s.%s', named '%s', already exists "
                     "on the model '%s' (tied to the field '%s.%s')")
            raise ImplementationError(error % (self._model.__name__, self.name, self.related_name,
                                      self.related_to, existing[0][1], existing[0][0]))