Django字段中的项目计数

Question

models.py

class Event(models.Model):
    name = models.CharField(max_length=20, unique=True)
    distance = models.IntegerField()
    date = models.DateField()

class Category(models.Model):
    name = models.CharField(max_length=20, unique=True)
    description = models.CharField(max_length=20, unique=True)
    isnew = models.BooleanField(default=False)

class Result(models.Model):
    event = models.ForeignKey(Event)
    category = models.ForeignKey(Category)
    score = models.IntegerField()

I want to do a query to return a count of each unique Category in the Result table, for a given Event.

What I'm doing now is something like:

results = Result.objects.filter(event=myevent)
categorycountdict = {}
for r in results:
    if r.category in categorycountdict:
        categorycountdict[r.category] += 1
    else:
        categorycountdict[r.category] = 1

Is there a better way, perhaps by query instead of python.

Answer 1

You can use annotate() with values() . This approach is shown in the docs for values() . To get the count for each category name, you could do:

from django.db.models import Count

categories = Result.objects.filter(
    event=myevent,
).order_by('category').values(
    'category__name'
).annotate(count=Count('category__name'))

This will return a list of dictionaries with keys category__name and count , for example:

[{'count': 3, 'category__name': u'category1'}, {'count': 1, 'category__name': u'category2'}]

You could convert this to a single dictionary by using a dictionary comprehension:

counts_by_category = {d['category__name']: d['count'] for f in categories}

Answer 2

Use annotate :

from django.db.models import Count

Results.objects.filter(event=some_event).annotate(Count('category'), distinct=True)

Answer 3

There is collections.Counter in python standard library:

results = Result.objects.filter(event=myevent).select_related('category')
c = Counter(r.category for r in results)

Now c is a dict-like object where keys are Category instances, and values are counts.

This option is not suitable for large datasets though, since it doesn't use database features. So if you have a lot of data, approaches that use values, annotate and Count are more suitable.

select_related is used here in order to eliminate database hit for every result. Basically it makes 1 query with join and generates python objects for categories.

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It turns out, that ManyToManyField keeps track of unique records only, so below answer is incorrect.

Your Event and Category models expose many-to-many connection through Result model.

You can express it using through in Django (note categories ManyToManyField):

class Event(models.Model):
    name = models.CharField(max_length=20, unique=True)
    distance = models.IntegerField()
    date = models.DateField()
    categories = models.ManyToManyField(Category, through='Result', related_name='events')

In your code, for given event you could query like this:

event.categories.count()

Taking Alasdair note into account:

event.categories.values('pk').annotate(count=Count('pk'))