Django: Safely check for related objects before deletion

I have a Django app with two models:

class Foo(models.Model):
    baz = models.CharField(max_length=100)

class Bar(models.Model):
    foo = models.ForeignKey(Foo)

In a view, I want to delete a Bar object. Currently it's done with:

foo_to_delete = Foo.objects.get(pk=1)
if not foo_to_delete.bar_set.exists(): # step 1
    foo_to_delete.delete() # step 2

But now if at the time between the step 1 and the step 2, someone saves a new Bar object with the foo field pointing to foo_to_delete object, this new Bar object would be deleted by step 2.

Is there any way to avoid this, maybe by doing a single SQL call ? Does the Django ORM provides something for doing this kind of check before deletion safely ?

Solution

I think I have solved the problem using models.PROTECT.

The models now looks like this:

class Foo(models.Model):
    baz = models.CharField(max_length=100)

class Bar(models.Model):
    foo = models.ForeignKey(Foo, on_delete=models.PROTECT)

And the view is like this:

foo_to_delete = Foo.objects.get(pk=1)
try:
    foo_to_delete.delete()
except models.ProtectedError:
    # show an error message

This seems to be mapped to only one SQL query, thus making it safe (I think).