Django queryset update() 以 += 方式

Question

How can I call update on Django queryset, that does not write particular value, but modify the value already there, as follows

Entry.objects.filter(pub_date__year=2010).update(count+=1)

How can I achieve that?

Answer 1

You can use F expression from docs:

reporter = Reporters.objects.filter(name='Tintin')

reporter.update(stories_filed=F('stories_filed') + 1)

Try this:

from django.db.models import F
Entry.objects.filter(pub_date__year=2010).update(count=F('count') + 1)

Answer 2

for e in Entry.objects.filter(pub_date__year=2010):
    e.count+= 1
    e.save()

Answer 3

Pay attention to:

if you are using F-expression - you are using .update() method.

If you have a needed logic in the overridden .save() method - it won't be called in such updating.

To use the .save() method you need to use a general approach:

foo.bar += 1
foo.save()

Docs :

Finally, realize that update() does an update at the SQL level and, thus, does not call any save() methods on your models, nor does it emit the pre_save or post_save signals (which are a consequence of calling Model.save()