是否可以在ReferenceFields上使用unique_with？

Question

我无法让unique_with在MongoEngine中使用ReferenceFields。我的模型看起来像这样：

class WorkoutSchedule(database.Document):
    """ Defines a workout schedule """
    user = database.ReferenceField(User)
    title = database.StringField(
        required=True,
        min_length=3,
        max_length=30,
        unique_with=user)

如果user和title相等，我希望文档是唯一的。但是，这引发了一个例外：

TypeError: Error when calling the metaclass bases
    'ReferenceField' object is not iterable

是否可以使用unique_with和ReferenceFields，还是必须手动解决此问题？

Answer 1

请参阅文档https://mongoengine-odm.readthedocs.org/en/latest/guide/defining-documents.html?highlight=unique_with#field-arguments：

unique_with (Default: None)
    A field name (or list of field names) that when taken together 
    with this field, will not have two documents in the collection
    with the same value.

因此unique_with必须是basestring或list个{1}}字段名称：

basestring