Question

I am looking to find a way to annotate a queryset with the counts of a subset of related items. Below is a subset of my models:

class Person(models.Model):
    Name = models.CharField(max_length = 255)
    PracticeAttended = models.ManyToManyField('Practice',
                                              through = 'PracticeRecord')

class Club(models.Model):
    Name = models.CharField(max_length = 255)
    Slug = models.SlugField()
    Members = models.ManyToManyField('Person')

class PracticeRecord(PersonRecord):
    Person = models.ForeignKey(Person)
    Practice = models.ForeignKey(Practice)

class Practice(models.Model):
    Club = models.ForeignKey(Club, default = None, null = True)
    Date = models.DateField()

I’m looking to make a queryset which annotates the number of club specific practices attended by a person. I can already find the total number of practices by that person with a query of Person.objects.all().annotate(Count('PracticeRecord'))

However I would like someway to annotate the number of practices that a person attends for a specific club.

I would prefer something using the django ORM without having to resort to writing raw SQL.

Thanks.

Answer 1

However I would like someway to annotate the number of practices that a person attends for a specific club.

Let us see.

First, find the specific club.

club = Club.objects.get(**conditions)

Next, filter all Persons who have practiced at this club.

persons = Person.objects.filter(practicerecord__Practice__Club = club)

Now, annotate with the count.

q = persons.annotate(count = Count('practicerecord'))

Edit

I was able to successfully make this work in my test setup: Django 1.2.3, Python 2.6.4, Postgresql 8.4, Ubuntu Karmic.

PS: It is a Good Idea™ to use lower case names for the fields. This makes it far easier to use the double underscore (__) syntax to chain fields. For e.g. in your case Django automatically creates practicerecord for each Person. When you try to access other fields of PracticeRecord through this field you have to remember to use title case.

If you had used lower case names, you could have written:

persons = Person.objects.filter(practicerecord__practice__club = club)
#                                               ^^        ^^  

which looks far more uniform.

PPS: It is Count('practicerecord') (note the lower case).