问题
I've a model
from django.contrib.auth.models import User
class ModelA(models.Model):
phone = models.CharField(max_length=20)
user = models.ForeignKey(User)
I need to insert the data into this model. I've an endpoint hosted which provides me the following data {'phone':XXXXXXXX, 'user_id':123}.
Now when I insert the data into this model like
obj = ModelA.objects.create(phone=data['phone'], user = data['user_id]
It throws an error saying
Cannot assign "u'123'": "ModelA.user" must be a "User" instance.
Agreed, since because with django orm you can interact in terms of objects and not numbers. Hence I first found the object of the User and then created ModelA object.
user_obj = User.objects.get(id=data['id']
modelobj = ModelA.objects.create(phone=data['phone'], user = user_obj
Till here its all working fine.
Now, my question is that is there any other way of assigning/creating ModelA object directly using user_id not User object, since it first involves quering User Model and then inserting. Its like an extra read operation for every ModelA object created.
回答1:
(I keep getting votes for an answer that is wrong. If the database you are using has actual integrity checking, you can just use attr_name_id to insert the related object.
Wish I could just delete it. Look at the answer below.)
Basically, you can't do that. Django's ORM is expecting an object and not just a key. It's the price to pay to have an extra layer of abstraction that is supposed to check for integrity at the application level, instead of the DB-level.
If performance is a problem (and in the general case it isn't), you can make the read operation of the User object as light as possible. The only function will return a queryset of objects, forcing the select to retrieve only the id column, which is surely indexed.
user = User.objects.only('id').get(id=data['user_id'])
obj = ModelA.objects.create(phone=data['phone'], user=user)
回答2:
As can be seen in my historic comments, the accepted answer is not really correct. So as @yekta requested I re-submit my comments:
To create the parent model, use integer value like:
ModelA.objects.create(phone=data['phone'], user_id=1)
来源：https://stackoverflow.com/questions/30017334/django-foreign-key-must-be-an-instance