Jeethu's Blog > Merging Django querysets

This one’s pretty basic (from the docs) , but I end up using it all the times. Being able to “AND” and “OR” django querysets can really simplify a lot of code. Here’s an instance (a simplification of the setup I have in tagz). Lets start by defining 2 models.

from django.db import models

class Tag( models.Model ) :
    text = models.CharField(max_length=255)

class Post( models.Model ) :
    link  = models.URLField(max_length=2048)
    title = models.CharField(max_length=255)
    tags  = models.ManyToManyField(Tag)

Now, lets say I want to get all Posts tagged as django or python.

qs = Post.objects.filter(tags__text='python') | Post.objects.filter(tags__text='django')

Now as intuitive as it might seem, using AND doesn’t seem to work.

qs = Post.objects.filter(tags__text='python') & Post.objects.filter(tags__text='django')
# qs.count() returns 0

So, we end up chaining filters like this:

qs = Post.objects.filter(tags__text='python').filter(tags__text='django')

Which boils down to simple for loop.

def filter_tags( tags ) :
    '''
    tags: a list of strings
    '''
    p = Post.objects.all()
    for t in tags :
        p = p.filter(tags__text=t)
    return p

Tags:django