django: using queryset chainable model manager

In django, you can use your custom model manager.

I usally add display() for filtering is_display and created_for_day() for filtering by created_at().

It’s really easy to add custom model manager.

Adding custom model manager

Add custom model manager in models.py (or anywhere you want), and add it to model.

class DisplayManager(models.Manager):
    def get_queryset(self):
        return super(DisplayManager, self).get_queryset().filter(is_display=True)

class Post(models.Model):
    # you have to add this for using default objects() manager
    objects = models.Manager()
    display = DisplayManager()
    ...
    is_display = models.BooleanField(default=True)
    ...

After that, you can simply use display for Post model.

>>> from post.models import Post
>>> Post.objects.count()
2
>>> Post.display.count()
1

Add custom model manager as method chain

But if you add custom model manager by that way, it has few problems: I can’t use both managers, and it’s not pythonic way at all.

So you can add custom manager chainable. Just add use_for_related_fields to custom model manager and set default model managers to custom one.

class DisplayManager(models.Manager):
    use_for_related_fields = True

    def get_queryset(self):
        return super(DisplayManager, self).get_queryset().filter(is_display=True)

class Post(models.Model):
    objects = DisplayManager()
    ...
    is_display = models.BooleanField(default=True)
    ...

Then you can use manager chainable.

>>> from post.models import Post
>>> Post.objects.count()
2
>>> Post.objects.published().count()
1

It looks great!!

However, it has a small problem. I can’t use this manager after queyset. What if you want to filter by display manger after get queryset? You can’t use your manager after queryset.

>>> from post.models import Post
>>> posts = Post.objects.all()
# I want 'posts.objects.display()' or sth

Use QuerySet and add it by as_manager

Here’s solution. You can use models.query.QuerySet instead of models.Manager. Then add it to model using as_manager

class BaseQuerySet(models.query.QuerySet):
    def display(self):
        queryset = self.filter(is_display=True)
        return queryset

class Post(models.Model):
    objects = BaseQuerySet.as_manager()
    ...
    is_display = models.BooleanField(default=True)
    ...

Then magic happens! You can use your new custom qureyset anytime!

>>> from post.models import Post
>>> posts = Post.objects.all()
>> posts.display().count()
1

Plus tip: inherit BaseQuerySet per models

If you have all managers per model, it’s hard to managing your code. So you can inherit BaseQuerySet that you make, and make new custom queryset for other models.

class PostQuerySet(BaseQuerySet):
    def deadline_for(self, day):
        today = datetime.date.today()
        td = today - datetime.timedelta(days=day)
        queryset = self.filter(deadline__gte=td).filter(is_display=True).order_by('-created_at')
        return queryset

class Post(models.Model):
    objects = PostQuerySet.as_manager()
    ...
    is_display = models.BooleanField(default=True)
    ...

Then you can use chainable manager whatever you want.

>>> from post.models import Post
>>> posts = Post.objects.all()
>> posts.display().count()
1
>> post_within_deadline = Post.objects.display().deadline(15)

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