Django: retrieving ManyToManyField objects with minimum set of queries - django

Question

My code looks like this:
models.py
class Tag(models.Model):
name = models.CharField(max_length=42)
class Post(models.Model):
user = models.ForeignKey(User, related_name='post')
#...various fields...
tags = models.ManyToManyField(Tag, null=True)
views.py
posts = Post.objects.all().values('id', 'user', 'title')
tags_dict = {}
for post in posts: # Iteration? Why?
p = Post.objects.get(pk=[post['id']]) # one extra query? Why?
tags_dict[post['id']] = p.tags.all()
How am I supposed to create a dictionary with tags for each Post object with minimum set of queries? Is it possible to avoid iterating, too?

Answer 1

Yes you will need a loop. But you can save one extra query in each iteration, you don't need to get post object to get all its tags. You can directly query on Tag model to get tags related to post id:
for post in posts:
tags_dict[post['id']] = Tag.objects.filter(post__id=post['id'])
Or use Dict Comprehension for efficiency:
tags_dict = {post['id']: Tag.objects.filter(post__id=post['id']) for post in posts}

Answer 2

If you have Django version >= 1.4 and don't really need a dictionary, but need to cut down the count of queries, you can use this method like this:
posts = Post.objects.all().only('id', 'user', 'title').prefetch_related('tags')
It seems to execute only 2 queries (one for Post and another for Tag with INNER JOIN).
And then you can access post.tags.all without extra queries, because tags was already prefetched.
{% for post in posts %}
{% for tag in post.tags.all %}
{{ tag.name }}
{% endfor %}
{% endfor %}