How can I get the distinct values of queryset?

Question

I have this query such as

query1 = A.objects.filter(a=a).values('color__red')

And the debugger told me that :

<QuerySet [{'color__red': 'yes'}, {'color__red': 'yes'}, {'color__red': 'yes'}, {'color__red': 'yes'}, {'color__red': 'yes'}, {'color__red': 'yes'}, {'color__red': 'yes'}, {'color__red': 'yes'}, {'color__red': 'yes'}, {'color__red': 'yes'}]>

I would like to count the number of distinct values of color__red (in my case it is one because color__red = 'yes' but sometimes it can be 2)

I know in the case of an arrays I have to do a len(list(set(array))) but I don't achieve to get the value in the case of queryset.

Could you help me please ?

Thank you !

Answer 1

You can add .distinct() [Django-doc] to the queryset:

query1 = A.objects.filter(a=a).values('color__red').distinct()

In case you want to count the number of distinct objects, you can use .count() [Django-doc] :

n_colors = A.objects.filter(a = a).values('color__red').distinct().count()

You can also define an .aggregate(..) [Django-doc] here, and work with a Count expression [Django-doc] here:

n_colors = A.objects.filter(a=a).aggregate(n_colors = Count('color__red', distinct=True))['n_colors']

The uniqueness filter will then be performed on the database level. So Django will construct a query like:

SELECT DISTINCT app_color.red
FROM app_a
LEFT OUTER JOIN app_color ON app_color.a_id = a.app_a.id
WHERE app_a.a = a

and for the .count() query:

SELECT COUNT(*)
FROM 
    (SELECT DISTINCT app_color.red
     FROM app_a
     LEFT OUTER JOIN app_color ON app_color.a_id = a.app_a.id
     WHERE app_a.a = a)

with the aggregate we get:

SELECT COUNT(DISTINCT app_color.red) AS n_colors
FROM app_a
LEFT OUTER JOIN app_color ON app_color.a_id = a.app_a.id
WHERE app_a.a = a

So this is done at the database, which is usually faster, and furthermore the amount of data that is transferred from the database to the application is significantly less.