Django DB Models F Combined Expression

Question

In [1]: from django.db.models import F

In [2]: from forum.models import Post

In [3]: post = Post.objects.get(id=1)

In [4]: post.view_count = F('view_count') + 1

In [5]: post.save()

In [6]: post.view_count

Out[6]: <CombinedExpression: F(view_count) + Value(1)>

In [7]: post = Post.objects.get(id=1)

In [8]: post.view_count

Out[8]: 3

After saved the post, it returned the combined expression.

I want the exact result (3).

is it possible to do without calling get method again / refresh_from_db ?

Answer 1

In Django 1.8+, you can use the refresh_from_db method. It won't save any SQL queries, but you might think the code is nicer.

>>> post = Post.objects.get(id=1)
>>> post.view_count
2
>>> post.view_count = F('view_count') + 1
>>> post.save()
>>> post.refresh_from_db()
>>> post.view_count
3

Since the update happens in the database, it's not possible to get the new value in Django without doing a get() or a refresh_from_db (which both cause a similar SQL query). However, it is possible to avoid the initial get() by using update() instead.

>>> Post.objects.filter(id=1).update(view_count=F('view_count') + 1)
>>> post = Post.objects.get(id=1)
>>> post.view_count
3

Answer 2

From the docs https://docs.djangoproject.com/en/1.7/ref/models/queries/#f-expressions,

Python never gets to know about encapsulated SQL expression post.view_count = F('view_count') + 1 # F('view_count') Django sql expression.

it is dealt with entirely by the database.

So changes done on Database while Python all knows about the expression you applied rather then result.

You can do it by this way directly:-

post = Post.objects.filter(id=1)
post.update(view_count = F('view_count') + 1)