Django annotate count with a distinct field

Question

I have two models defined loosely like this:

class InformationUnit(models.Model):     username = models.CharField(max_length=255)     project = models.ForeignKey('Project')     ...  class Project(models.Model):     name = models.CharField(max_length=255) 

Now, in a view, I want to annotate all the InformationUnits that belong to a project, so I do this:

p = Project.objects.all().annotate(Count('informationunit') 

which works just ok.

Furthermore, I want to know, in each project, how many distinct usernames participate. That is, count how many distinct usernames are there in the InformationUnits that compose one project. I have tried the following, but it simply counts the number of InformationUnit, regardless of the username:

p = Project.objects.all().annotate(Count('informationunit__username') 

Note that username is not an object, it is a string. Is there a clean way to do this or should I create a more complicated code based on loops and spaghetti code :P

Thanks a lot!

Answer 1

Count can take a distinct argument, like so:

p = Project.objects.all().annotate(Count('informationunit__username',                                           distinct=True)) 

This doesn't seem to be documented, but you can find it in the source for Count.

Answer 2

If you just want to count the distinct values, you can use the distinct() and count() functions:

count = Project.objects.values('informationunit__username').distinct().count()