django how to get count for manytomany field

问题

I have model for question:

class Question(models.Model):
    user = models.ForeignKey(User)
    title = models.CharField(max_length=120)
    description = models.TextField()
    answers = models.ManyToManyField('Answer',related_name='answer_name', blank=True)
    post_date = models.DateTimeField(auto_now=True)

def __unicode__(self):
    return self.title

And I have model for answer:

class Answer(models.Model):
    user = models.ForeignKey(User)
    question = models.ForeignKey(Question)
    ans_body = models.TextField()
    post_date = models.DateTimeField(auto_now=True)

def __unicode__(self):
    return self.ans_body

Question creation and answer submission are working perfectly. I cant correctly show answer for particular question. But when I try to get the count of answer for particular question its not showing. It displays 0 count.

In my view I am getting the list of the answer by:

context["question_list"] = Question.objects.all()

And in my template

{% for question in question_list %}
   {{ question.title }}
    Ans:{{question.answers.count}}
{% endfor %}

When I do this I get the count 0 if there are answers. How can I get the count of the answers for particular questions.

回答1:

This worked:

{{question.answer_set.count}}

Happy..

回答2:

You can do something like {{ question.answers.all.count }}, but if you are iterating over more than question it will cause a database query for every question.

If you want to annotate the whole queryset with the count for each question:

from django.db.models import Count

context['question_list'] = Question.objects.all().annotate(
    answer_count=Count('answers')
)

Then you can access the count for each question with {{ question.answer_count }}.

回答3:

Why not use: Question.objects.all().count()

For my project, I have a Field in 'Info' Model

users_like = models.ManyToManyField(settings.AUTH_USER_MODEL, related_name="%(app_label)s_%(class)s_likes", blank=True)

I use below code to count the number of like then show it in Admin List Page.

# admin.py
from django.contrib import admin

from .models import Info
class InfoAdmin(admin.ModelAdmin):
    list_display = ('id', 'title', 'like_count',)
    def like_count(obj):
        return obj.users_like.all().count()

admin.site.register(Info, InfoAdmin)

The result is:

Hope these can help you!

来源：https://stackoverflow.com/questions/27149984/django-how-to-get-count-for-manytomany-field

标签

python

django

django-queryset