Django : Filter query based on custom function

问题:

I've got a function built into my Django model class and I want to use that function to filter my query results.

  class service:
       ......
       def is_active(self):
            if datetime.now() > self.end_time:
                  return False
            return True

Now I want to use this function into my query filter, something like

nserv = service.objects.filter(is_active=True)

I know, for this simple 'is_active' case, I can directly make this comparision in filter query, but for more complex situations, that may not be possible. How should I make a query, based on custom functions?

回答1:

I would suggest you to use a custom manager for your class, like this you could use :

nserv = service.objects.are_active()

This would be achieved with something like:

class ServiceManager(models.Manager):
    def are_active(self):
        # use your method to filter results
        return you_custom_queryset

See custom managers

回答2:

You may not be able to, instead you can post-process the queryset with a list comprehension or generator expression.

For example:

[x for x in Q if x.somecond()]

回答3:

I just had a similar issue. The problem was i had to return a QuerySet instance. A quick solution for me was to do something like:

active_serv_ids = [service.id for service in Service.objects.all() if service.is_active()]
nserv = Service.objects.filter(id__in=active_serv_ids)

pretty sure this is not the prettiest and performant way to do this, but i works for me.

a more verbose way of doing this would be:

active_serv_ids = []

for service in Service.objects.all():
if service.is_active():
    active_serv_ids.append(service.id)

nserv = Service.objects.filter(id__in=active_serv_ids)

回答4:

The answer by Ignacio is interesting, but it does not return a queryset. This one does:

def users_by_role(role):
    users = User.objects.all()
    ids = [user.id for user in users if user.role == role]
    return users.filter(id__in=ids)