Delayed effect of calling set on ManyToManyField

[jeberger]

When calling set on a ManyToManyField, the effect is not visible in the current object, but is only visible after the object has been re-fetched from the DB:

my_model = MyModel.objects.get (id = 1)

my_model.some_m2m_field.set (some_list)
my_model.save()    # Not really necessary according to the docs

# These two lines should print the same thing, but the second one
# prints an empty list
print some_list
print my_model.some_m2m_field.all()

my_model = MyModel.objects.get (id = 1)
# Now the list is correct
print my_model.some_m2m_field.all()

[Tim Graham]

The behavior you describe seems to contradict ​the tested behavior.

Can you provide a failing test for Django's test suite that demonstrates the issue? Possibly you're using prefetch_related() similar to what's described in #26706.