How to use add(), set() and clear() methods of ManyRelatedManager in Django ManyToManyField with a through class?

Question

I have models as follows,

Class Bank(models.Model):
    customers = models.ManyToManyField(Customer,'banks', through='bank_customer')

Class Customer(models.Model):
    name = models.CharField(max_length=50)

Class Bank_customer(models.Model):
    customer = models.ForeignKey(Customer,on_delete=models.CASCADE)
    bank = models.ForeignKey(Bank,on_delete=models.CASCADE)
    city = models.ForeignKey(City,on_delete=models.CASCADE)
    
Class City(models.Model):
    name = models.CharField(max_length=100)

How do I add Customer objects to Bank? The following does not work

bank.customers.add(customer)

Here bank and customer are saved instances of their classes. Doing this violates not_null constraint for city ForeignKey in Bank_customer table.

https://docs.djangoproject.com/en/3.0/topics/db/examples/many_to_many/ — iklinac, Jul 30 '20 at 14:41
@iklinac This link does not explain how many_to_many works with the through model. It only explains the simple manytomany relationship. — Vikramark, Jul 30 '20 at 14:55
https://docs.djangoproject.com/en/3.0/topics/db/models/#extra-fields-on-many-to-many-relationships — iklinac, Jul 30 '20 at 14:57
@iklinac Thanks, this does explain, the add() and clear() methods. But, in the set() method, it only allows for same ```through_fields``` for the set of through classes. How do I use set() with unique through_fields for each of the related objects in the list? — Vikramark, Jul 30 '20 at 15:17

Guzman Ojero · Answer 1 · 2021-03-28T03:08:07.797

First, some general comments. I'm no expert, but I would make this changes to the models to start.

Be carefull with the reserved word class, it must be all lowercase.
When you define the classes, the order is important. The classes that inherit or are backward-related must be defined last. (eg: Bank must be defined after Customer, if not you'll have an error like "Customer is not defined" or something like that).
I think that you may need a name attribute/field in the Bank model. I mean, banks have a name.
In the Bank class I think it's better to explicitly use the related name keyword: related_name='banks'. It makes readability better.
It's more Pythonic to name classes using the CapWords convention (PEP 8). Use BankCustomer instead of Bank_customer model.

About your specific question.

I did this:

c1 = Customer(name="Guido van Rossum")
c1.save()
b1 = Bank(name="Bank of America")
b1.save()

Then I did b1.customers.add(c1) and this error was raised: IntegrityError: NOT NULL constraint failed: app2_bankcustomer.city_id. It seems that it was expecting city_id to have null=True in the BankCustomer.

I continued with:

ct1 = City('New York')
ct1.save()
bc1 = BankCustomer(customer=c1, bank=b1, city=ct1)
bc1.save()

And then when I did b1.customers.add(c1) no errors were raised.

The problem is the IntegrityError error, apparently:

django.db.utils.IntegrityError: NOT NULL --> this is a very common error for beginners. It means that you have a field that is null=False (aka the default) and you're trying to create that model WITHOUT that field having any data. (source)

So you can make a little change to the BankCustomer. You add null=True in the city field.

class BankCustomer(models.Model):
    customer = models.ForeignKey(Customer, on_delete=models.CASCADE)
    bank = models.ForeignKey(Bank, on_delete=models.CASCADE)
    city = models.ForeignKey(City, null=True, on_delete=models.CASCADE)

    def __str__(self):
        return f'{self.customer} - {self.bank} - {self.city}'

(Don't forget to run makemigrations and migrate)

After this, no errors are raised.

Some other sources that discuss the issue:

django.db.utils.IntegrityError: NOT NULL constraint failed: app.area_id

django.db.utils.IntegrityError: NOT NULL constraint failed: products_product.image ERROR WITH IMAGE FIELD

https://code.djangoproject.com/ticket/21783

https://forum.djangoproject.com/t/polls-not-null-constraint-failed/4396/2

https://github.com/sibtc/django-beginners-guide/issues/20

If you check the admin you'll see that the Bank model only accepts name and no customers.

Which I think is logical because you want to use the BankCustomer (in your code: Bank_customer).

I hope this helps.

score 2 · Answer 2 · answered Nov 03 '21 at 21:22

I can't find the django doc for it. however looking at the Django3.1's source code of ManyRelatedManager__add, it accepts through_defaults parameter

def add(self, *objs, through_defaults=None):
...
                self._add_items(
                    self.source_field_name, self.target_field_name, *objs,
                    through_defaults=through_defaults,
                )
...

so, instead of

bank.customers.add(customer)

try

bank.customers.add(customer, through_defaults={'city_id':SOME_CITY_OBJ_ID_YOU_WANT})

this works on .set() function as well

How to use add(), set() and clear() methods of ManyRelatedManager in Django ManyToManyField with a through class?

2 Answers2