Checking for object’s existence in ManyToMany relation (Django)

djangodjango-models

I'd like to check for a particular object's existence within a ManyToMany relation. For instance:

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

class B(models.Model):
    pass

results = [some query]

for r in results:
    print r.has_object // True if object is related to some B of pk=1

My first stab at [some query] was A.objects.all().annotate(Count(has_object='members__id=1')) but it looks like I can't put anything more than the field name into the argument to Count. Is there some other way to do this?

Best Answer

You can try

A.objects.filter(members__id=1).exists()

Related Solutions

Filter ManyToMany box in Django Admin

Ok, this is my solution using above classes. I added a bunch more filters to filter it correctly, but I wanted to make the code readable here.

This is exactly what I was looking for, and I found my solution here: http://www.slideshare.net/lincolnloop/customizing-the-django-admin#stats-bottom (slide 50)

Add the following to my admin.py:

class CustomerForm(forms.ModelForm): 
    def __init__(self, *args, **kwargs):
        super(CustomerForm, self).__init__(*args, **kwargs)
        wtf = Category.objects.filter(pk=self.instance.cat_id);
        w = self.fields['categories'].widget
        choices = []
        for choice in wtf:
            choices.append((choice.id, choice.name))
        w.choices = choices


class CustomerAdmin(admin.ModelAdmin):
    list_per_page = 100
    ordering = ['submit_date',] # didnt have this one in the example, sorry
    search_fields = ['name', 'city',]
    filter_horizontal = ('categories',)
    form = CustomerForm

This filters the "categories" list without removing any functionality! (ie: i can still have my beloved filter_horizontal :))

The ModelForms is very powerful, I'm a bit surprised it's not covered more in the documentation/book.

Django ManyToMany filter()

Just restating what Tomasz said.

There are many examples of FOO__in=... style filters in the many-to-many and many-to-one tests. Here is syntax for your specific problem:

users_in_1zone = User.objects.filter(zones__id=<id1>)
# same thing but using in
users_in_1zone = User.objects.filter(zones__in=[<id1>])

# filtering on a few zones, by id
users_in_zones = User.objects.filter(zones__in=[<id1>, <id2>, <id3>])
# and by zone object (object gets converted to pk under the covers)
users_in_zones = User.objects.filter(zones__in=[zone1, zone2, zone3])

The double underscore (__) syntax is used all over the place when working with querysets.

Best Answer

Related Solutions

Filter ManyToMany box in Django Admin

Django ManyToMany filter()

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