How can I use the F() object to do this with the Django ORM?

Question

I encountered a model like this:

class Task(models.Model):
    timespan = models.IntegerField(null=True, blank=True)

class Todo(models.Model):
    limitdate = m         


        

Answer 1

The main issue is that DB does not support date + integer and its hard to write ORM query to date + integer::interval, for PostgreSQL for example, where integer is the value of the task_timespan column, in days count.

However, as
limitdate <= today + task__timespan equals to
limitdate - today <= task__timespan

We could transform the query to

Todo.objects.filter(task__timespan__gte=F('limitdate') - today).distinct()

thus the SQL becomes something like integer >= date - date, that should work in PostgreSQL because date - date outputs interval which could be compared w/ integer days count.

In other DBs such as SqLite, it's complicated because dates need to be cast w/ julianday() at first...and I think you need to play w/ extra() or even raw() to get the correct SQL.

Also, as Chris Pratt suggests, if you could use timestamp in all relative fields, the query task might become easier because of less limited add and subtract operations.

P.S. I don't have env to verify it now, you could try it first.