How to use custom manager with related objects?

Question

I have a custom manager. I want to use it for related objects. I found use_for_related_fields in docs. But it does not work the way I used it:

class RandomQueryset(models.query.QuerySet):

    def randomize(self):       
        count = self.count()
        random_index = random.randint(0, count - 1)
        return self.all()[random_index]


class RandomManager(models.Manager):

    use_for_related_fields = True

    def get_query_set(self):
        return RandomQueryset(self.model, using=self._db)

    def randomize(self):
        return self.get_query_set().randomize()

I used it for one model:

>>> post = PostPages.default_manager.filter(image_gallery__isnull=False).distinct().randomize()

And tried to do the same with m2m related object:

>>> post.image_gallery.randomize()

Got an error:

AttributeError: 'ManyRelatedManager' object has no attribute 'randomize'

Is it possible to use a custom manager in the way I did it? If so, how do you make it work?

Edit

My models:

class ShivaImage(models.Model, ImageResizing):
    image = models.ImageField(upload_to='img')
    slide_show = models.BooleanField() 
    title = models.CharField(max_length=100)
    text = models.TextField(max_length=400)
    ordering = models.IntegerField(blank=True, null=True)

    objects = RandomManager()


class PostPages(models.Model):
    image_gallery = models.ManyToManyField(ShivaImage, blank=True,
                                       related_name='gallery',)
    # all the other fields... 

    objects = RandomManager()

Answer 1

Setting use_for_related_fields to True on the manager will make it available on all relations that point to the model on which you defined this manager as the default manager. This is documented here

class MyManager(models.Manager):
    use_for_related_fields = True
    # ...

I suppose you have it only enabled on your PostPages model, not on your Gallery model (or whatever the model is called that is referenced through post_image_gallery). If you want to have additionally functionality on this realtion manager you need to add a custom default manager with use_for_related_fields = True to your Gallery model!

Answer 2

For completeness of the topic, Django 1.7 (finally) supports using a custom reverse manager, so you can do something like that (just copying from the django docs):

from django.db import models

class Entry(models.Model):
    objects = models.Manager()  # Default Manager
    entries = EntryManager()    # Custom Manager

b = Blog.objects.get(id=1)
b.entry_set(manager='entries').all()

Answer 3

In django 2.0 use_for_related_fields is deprecated https://docs.djangoproject.com/en/2.0/releases/1.10/#manager-use-for-related-fields-and-inheritance-changes

You should use base_manager_name: https://docs.djangoproject.com/en/2.0/ref/models/options/#django.db.models.Options.base_manager_name

Updated docs: https://docs.djangoproject.com/en/2.0/topics/db/managers/#using-managers-for-related-object-access

class MyModel(models.Model):
    field1 = ...
    field2 = ...
    special_manager = MyManager()

    class Meta:
        base_manager_name = 'special_manager'

Answer 4

Also, in the custom manager, make sure to access the queryset via the self.get_query_set() proxy method when implementing custom filters to be called from related manager:

class EventManager(models.Manager):

    use_for_related_fields = True

    def visible_events(self):
        today = datetime.date.today()
        # don't do this !!! 
        # unsuitable for related managers as could retrieve extraneous objects
        # qs = super(EventManager, self).get_query_set()
        # Use queryset proxy method as follows, instead:
        qs = self.get_query_set()
        qs = qs.filter(visible_from__lte=today, visible_to__gte=today)
        return qs


class Event(models.Model):

    visible_from = models.DateField(_(u'visible from'), null=False, blank=False)
    visible_to = models.DateField(_(u'visible to'), null=False, blank=False)
    concepts = models.ManyToManyField(Concept, through='ConceptEventRegistration')

    objects = EventManager()

Sample usage:

my_concept = Concept.objects.get(id=1)
# retrieve all events related to the object
my_concept.event_set.all()
# retrieve all visible events related to the object
my_concept.event_set.visible_events()