Creating custom Field Lookups in Django

Question

How do you create custom field lookups in Django?

When filtering querysets, django provides a set of lookups that you can use: __contains, __iexact, __in, and so forth. I want to be able to provide a new lookup for my manager, so for instance, someone could say:

twentysomethings = Person.objects.filter(age__within5=25)

and get back all the Person objects with an age between 20 and 30. Do I need to subclass the QuerySet or Manager class to do this? How would it be implemented?

Answer 1

Rather than creating a field lookup, best practice would be to create a manager method, that might look a little bit like this:

class PersonManger(models.Manager):
    def in_age_range(self, min, max):
        return self.filter(age__gte=min, age__lt=max)

class Person(models.Model):
    age = #...

    objects = PersonManager()

then usage would be like so:

twentysomethings = Person.objects.in_age_range(20, 30)

Answer 2

A more flexible way to do this is to write a custom QuerySet as well as a custom manager. Working from ozan's code:

class PersonQuerySet(models.query.QuerySet):
    def in_age_range(self, min, max):
        return self.filter(age__gte=min, age__lt=max)

class PersonManager(models.Manager):
    def get_query_set(self):
         return PersonQuerySet(self.model)

    def __getattr__(self, name):
        return getattr(self.get_query_set(), name)

class Person(models.Model):
    age = #...

    objects = PersonManager()

This allows you to chain your custom query. So both these queries would be valid:

Person.objects.in_age_range(20,30)

Person.objects.exclude(somefield = some_value).in_age_range(20, 30)