Question

In Django, if I have a model class, e.g.

from django.db import models

class Transaction(models.Model):
    ...

then if I want to add methods to the model, to store e.g. reasonably complex filters, I can add a custom model manager, e.g.

class TransactionManager(models.Manager):

    def reasonably_complex_filter(self):
        return self.get_query_set().filter(...)


class Transaction(models.Model):
    objects = TransactionManager()

And then I can do:

>>> Transaction.objects.reasonably_complex_filter()

Is there any way I can add a custom method that can be chained to the end of a query set from the model?

i.e. add the custom method in such a way that I can do this:

>>> Transaction.objects.filter(...).reasonably_complex_filter()

Answer 1

You need to add methods to the QuerySet which you eventually end up with. So you need to create and use a QuerySet subclass which has the methods you define wherever you want this functionality.

I found this tutorial which explains how to do it and reasons why you might want to:

https://web.archive.org/web/20160329131857/http://adam.gomaa.us/blog/2009/feb/16/subclassing-django-querysets/index.html