Django: Use query result in future queries instead of repeating it

Question

I have a custom user profile model. This model has can_edit property that uses ContentType and Permission objects to determine if the user has permission or not. I'm using this property within serializer and it works fine, but is terribly inefficient, as per each user the ContentType and Permission are queried again.

It seems like prefetch_related could fit here, but I don't know how to apply it, as these objects are not referenced directly through some properties, but kinda queried separately. Is it possible to fetch the ContentType and Permission beforehand and just use the results in further queries?

Here's my model:

class CustomProfile(models.Model):
    user = models.OneToOneField(User, related_name="profile", on_delete=models.CASCADE)

    @property
    def can_edit(self):
        content_type = ContentType.objects.get_for_model(Article)
        permission, _ = Permission.objects.get_or_create(
            codename="edit", name="Can edit", content_type=content_type
        )
        return self.user.has_perm(self._permission_name(permission))

    def _permission_name(self, permission):
        return f"{permission.content_type.app_label}.{permission.codename}"

My current query:

User.objects.order_by("username").select_related("profile")

My serializer:

class UserSerializer(serializers.ModelSerializer):
    can_edit = serializers.ReadOnlyField(source="profile.can_edit")

    class Meta:
        model = User
        fields = (
            "id",
            "username",
            "first_name",
            "last_name",
            "is_active",
            "is_staff",
            "can_edit",
        )

In fact, I have more than one property with similar content to can_edit, so each user instance adds around 6 unnecessary queries for ContentType and Permission.

How can I optimize it?

not sure if this is what you want but checkout cached_property docs.djangoproject.com/en/3.1/ref/utils/… — alex
– alex, Commented Oct 5, 2020 at 12:58
no, there are multiple objects, each of them will call its own property which might have different value, and all of them are repeating the same queries underneath — Djent
– Djent, Commented Oct 5, 2020 at 13:12
", I have more than one property with similar content to can_edit" On this UserSerializer class or some other class? @Djent — JPG
– JPG, Commented Oct 7, 2020 at 13:01
@Djent Can you add an example of it? (maybe I can provide an accurate solution) — JPG
– JPG, Commented Oct 8, 2020 at 8:09

JPG · Accepted Answer · 2020-10-08 08:27:36Z

2

+150

Actually, you don't have to reinvent the wheel, Django already has done it for us.

Here change your can_edit(...) as below.

class CustomProfile(models.Model):
    user = models.OneToOneField(User, related_name="profile", on_delete=models.CASCADE)

    @property
    def can_edit(self):
        return self.user.has_perm("app_name_article.can_edit")

Here, the user.has_perm()--(Django Doc) is beautifully built in an efficient way

Note

I don't think we need to find out the <app_label>_<model_name> string in a "programmatic" way.

edited Oct 8, 2020 at 8:27

answered Oct 8, 2020 at 8:20

JPG

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3 Comments

Lord Elrond Over a year ago

This is surely the "recommended" solution! It's much better to cache the result of a check rather than caching the steps necessary to perform said check (especially when it's builtin). Do note that this comes with caveats, for example, you will need to repopulate the cache if you are programmatically adding a permission to a user, then immediately checking it. See the docs on **Permission Caching for more info.

Djent Over a year ago

Thanks, it makes sense. I mean, I still kinda need to get the permission names programmatically, but I can cache them as a class attribute for CustomProfile.

JPG Over a year ago

@Djent "....I still kinda need to get the permission names programmatically....", Unfortunately, I don't see any such situation in this given context. IMHO, use hard-coded strings instead of programmatic way (in this situation)

user13006228 · Accepted Answer · 2020-10-07 19:46:52Z

0

Use a __init__ special method to initialize any propertys/variables that you need to use more then once.

Example:

class CustomProfile(models.Model):
   user = models.OneToOneField(User, related_name="profile", on_delete=models.CASCADE)
   
   def __init__(self, *args, **kwarg):
       super().__init__(*args, **kwargs)
       self.content_type = ContentType.objects.get_for_model(Article)
       self.permission, _ = Permission.objects.get_or_create(codename="edit", name="Can edit", content_type=content_type)

    @property
    def can_edit(self):
       return self.user.has_perm(self._permission_name(self.permission))

Now you can use these values in your methods.

answered Oct 7, 2020 at 19:46

user13006228

1 Comment

Djent Over a year ago

But it still will be repeated multiple times as the multiple profile objects will be instantiated.

lampslave · Accepted Answer · 2020-10-11 11:34:14Z

0

Use @cached_property instead of @property to cache can_edit.
https://docs.djangoproject.com/en/3.1/ref/utils/#django.utils.functional.cached_property

answered Oct 11, 2020 at 11:34

lampslave

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Comments

minglyu · Accepted Answer · 2020-10-11 11:34:47Z

0

For optimizing the query, you can use prefetch_related in combination with Case...When Conditional Expressions

from django.db.models import Case, When, BooleanField

qs = User.objects.prefetch_related('user_permissions').annotate(
      can_edit = Case(
      When(user_permissions__codename='edit_article', then=True),
      default=False, 
      output_field=BooleanField())
     )

# check the results
>>> [(i.can_edit, i.username) for i in qs]

edited Oct 11, 2020 at 11:34

answered Oct 11, 2020 at 11:24

minglyu

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