Making queries using F() and timedelta at django

Question

I have the following model:

class Process(models.Model):
  title = models.Charfield(max_length=255)
  date_up = models.DateTimeField(auto_now_add=True)
  day         


        

Answer 1

I tried to use solution by Lutz Prechelt above, but got MySQL syntax error. It's because we can't perform arithmetic operations with INTERVAL in MySQL.

So, for MySQL my solution is create a custom DB function:

class MysqlSubDate(Func):
    function = 'SUBDATE'
    output_field = DateField()

Example of usage:

.annotate(remainded_days=MysqlSubDate('end_datetime', F('days_activation')))

Also you can use timedelta, it will be converted into INTERVAL

.annotate(remainded_days=MysqlSubDate('end_datetime', datetime.timedelta(days=10)))