uMongo unique issue

umongo_test.py

import asyncio
from datetime import datetime
from motor.motor_asyncio import AsyncIOMotorClient
from umongo import Instance, Document, fields, validate

db = AsyncIOMotorClient().test
instance = Instance(db)

@instance.register
class User(Document):
    email = fields.EmailField(required=True, unique=True)
    birthday = fields.DateTimeField(validate=validate.Range(min=datetime(1900, 1, 1)))
    friends = fields.ListField(fields.ReferenceField("User"))

    class Meta:
        collection = db.user

loop = asyncio.get_event_loop()

async def main():
    goku = User(email='goku@sayen.com', birthday=datetime(1984, 11, 20))
    await goku.commit()
    vegeta = User(email='vegeta@over9000.com', friends=[goku])
    await vegeta.commit()
    try:
        goku2 = User(email='goku@sayen.com', birthday=datetime(1984, 11, 20))
        await goku2.commit()
    except Exception as ex:
        print(ex)

    print(vegeta.friends)
    # <object umongo.data_objects.List([<object umongo.dal.pymongo.PyMongoReference(document=User, pk=ObjectId('5717568613adf27be6363f78'))>])>
    print(vegeta.dump())
    # {id': '570ddb311d41c89cabceeddc', 'email': 'vegeta@over9000.com', friends': ['570ddb2a1d41c89cabceeddb']}

    data = User.find({"email": 'vegeta@over9000.com'})
    async for user in User.find():
        print(user)

loop.run_until_complete(main())