Detect duplicate inserts when adding many-to-many relation

Question

Let's assume there are two models, A and B:

class A(models.Model):
    name = models.CharField(max_length=100)


class B(models.Model):
    children = models.ManyToManyField(A)

I'm using b.children.add() method to add instance of A to b:

a = A.objects.get(pk=SOMETHING)
b.children.add(a)

As far as I know, Django by default doesn't allow duplicate many-to-many relationship. So I cannot add same instance of A more than once.

But the problem is here, I fetch instances of A with another query, then loop around them and add them one by one. How can I detect a duplicate relation? Does add() method return something useful?

Answer 1

A look at the source code reveals that Django first checks to see if there are any entries that already exist in the database, and then only adds the new ones. It doesn't return any information to the caller, though.

It's not clear if you actually need to detect duplicates, or if you just want to make sure that they're not being added to the database? If it's the latter then everything's fine. If it's the former, there's no way around hitting the database. If you're really concerned about performance you could always perform the check and update the through table yourself (i.e. re-implement add()).