How to check if ManyToMany field is not empty?

Question

How do I check if there are any ManyToMany field objects related to my model object?

For example, I have a model:

class Category(models.Model):
    related_categories = models.ManyToManyField('self', blank=True)

I want to do something only if there are related objects existing:

if example_category.related_categories:
    do_something()

I tried to do example_category.related_categories, example_category.related_categories.all(), example_category.related_categories.all().exists(), example_category.related_categories.count(), but none of these works for me.

I have no any additional conditions to filter by.

Is there any easy way to check emptiness of this field?

Mcgee

What is happening if you do that, this looks like the correct solution. Note however that here your related_categories is symmetrical. So that means if a is related to b, then b is related to a. – Cruz

.exists() would the right solution here. – Prue

@WillemVanOnsem it looks like my condition is always true, even when there are no related objects. Can the blank argument cause this, maybe? – Mcgee

@Dibidalidomba: no... blank actually has impact on the forms. But as said before, if you add a to the related_categories of b, then this means that the related_categories of a are non-empty as well. – Cruz

Answer 1

you should use the .exists method:

related_categories = example_category.related_categories
if related_categories.exists():
    # do something

Etienne

Answer 2

This works for me:

if hasattr(example_category, "related_categories"):
    ...

Wellfixed