Getting a count of objects in a queryset in Django

How can I add a field for the count of objects in a database.开发者_JAVA技巧 I have the following models:

class Item(models.Model):
    name = models.CharField()

class Contest(models.Model);
    name = models.CharField()

class Votes(models.Model):
    user = models.ForeignKey(User)
    item = models.ForeignKey(Item)
    contest = models.ForeignKey(Contest)
    comment = models.TextField()

To find the votes for contestA I am using the following query in my view

current_vote = Item.objects.filter(votes__contest=contestA)

This returns a queryset with all of the votes individually but I want to get the count votes for each item, anyone know how I can do that? thanks

To get the number of votes for a specific item, you would use:

vote_count = Item.objects.filter(votes__contest=contestA).count()

If you wanted a break down of the distribution of votes in a particular contest, I would do something like the following:

contest = Contest.objects.get(pk=contest_id)
votes   = contest.votes_set.select_related()

vote_counts = {}

for vote in votes:
  if not vote_counts.has_key(vote.item.id):
    vote_counts[vote.item.id] = {
      'item': vote.item,
      'count': 0
    }

  vote_counts[vote.item.id]['count'] += 1

This will create dictionary that maps items to number of votes. Not the only way to do this, but it's pretty light on database hits, so will run pretty quickly.

Another way of doing this would be using Aggregation. You should be able to achieve a similar result using a single query. Such as this:

from django.db.models import Count

Item.objects.values("contest").annotate(Count("id"))

I did not test this specific query, but this should output a count of the items for each value in contests as a dictionary.

Use related name to count votes for a specific contest

class Item(models.Model):
    name = models.CharField()

class Contest(models.Model);
    name = models.CharField()

class Votes(models.Model):
    user = models.ForeignKey(User)
    item = models.ForeignKey(Item)
    contest = models.ForeignKey(Contest, related_name="contest_votes")
    comment = models.TextField()

>>> comments = Contest.objects.get(id=contest_id).contest_votes.count()

You can use len() to get the count of contestA's votes:

current_vote = len(Item.objects.filter(votes__contest=contestA))

Actually, len() is used with select_for_update() as shown below:

current_vote = len(Item.objects.select_for_update().filter(votes__contest=contestA))

Because select_for_update() doesn't work with count() as shown below:

current_vote = Item.objects.select_for_update().filter(votes__contest=contestA).count()

You can see my answer explaning about select_for_update() with count() or len().