How to make Django to increase a field by db value?

Question

For example, I have model:

class CacheMetaData(models.Model):
    count = models.IntegerField(default=0)

def inc(self):
    self.count += 1
    self.save()

If I define method inc() like above, then Django will execute a SQL statement like:

UPDATE the_table set count = 1 where id = 1

This way lead to race condition so that count might not be correctly increased. I want Django to execute:

update the_table set count = count + 1

So that count will always be correctly increased.

Can Django do that? If yes, how to define the field count?

Answer 1

according to django documentation you can use F expression like below:

from django.db.models import F

class CacheMetaData(models.Model):
    count = models.IntegerField(default=0)

    def inc(self):
        self.count = F("count") + 1
        self.save()

in your version django increases count memory value by 1 and set it to database and can lead to race condition but in this version increasing process executes by database and the database value of count increases by 1.

note: if you want to use count value in your code, you need to run self.refresh_from_db() to update memory from db.