Need a save() call after assigning a many to many relation

[Victor Porton]

As ​http://guido.vonrudorff.de/django-manytomany-relationship-and-unsaved-objects/ teaches (and our tests show too), save() should be called twice when using m2m fields:

parts = BarClass.objects.filter(label__in=['eggs', 'bacon', 'spam'])
foo = FooClass(label='lunch')
foo.save() # do not forget this line
foo.child = parts
foo.save()

It looks silly and counter-intuitive to call save() twice. Is this a bug?

At least the docs should say this vividly. So at very least we have a bug in documentation for not addressing this issue.

The model code used:

class BarClass(models.Model):
    label = models.CharField()
 
class FooClass(models.Model):
    label = models.CharField()
    child = models.ManyToManyField(BarClass)

[Tim Graham]

The blog post is incorrect. A second save isn't needed. Also, that code won't work in Django 2.0 and later which removed support for ​direct assignment to many to many relations.

The blog post should use foo.child.set(parts) as described ​in the documentation.

In the future, please use our support channels to ask "is it a bug?" questions.