Retrieving count of unique Django objects based on multiple fields of another Django model

Question

I have a Django model defined as follows:

class Session(models.Model):
    ...
    leader = models.ForeignKey(User, related_name='leader')
    follower = models.ForeignKey(User, related_name='follower')
    ...

Now let's say I have a QuerySet of Session objects:

some_sessions = Session.objects.filter(some_criteria=True)

How can I get a simple count of unique Users that have been either a leader or follower in any Session in some_sessions?

For example, a particular User may have been a leader in some number of Session objects, but a follower in a handful of other Session objects. That particular User should only contribute 1 to the desired unique Users count.

There are of course some naive solutions to this, but for performance reasons, I'm wondering if there's a solution that leverages the power of QuerySets a bit more, if possible.

EDIT: To clarify, here's a naive method that gives the result I need:

active_user_ids = set()
some_sessions = Session.objects.filter(some_criteria=True)
for session in some_sessions:
    active_user_ids.add(session.leader.id)
    active_user_ids.add(session.follower.id)
all_active_users = User.objects.filter(id__in=active_user_ids).count()

Try: User.leader_set.all().count() and User.follower_set.all().count() — bmons
– bmons, Commented Nov 19, 2019 at 6:22
@user6837382 thank you, but this would not get me the count that I want. For example, if users 1-100 are leaders and users 51-150 are followers, your method would result in a total count of 200, not the desired 150. — sunw
– sunw, Commented Nov 19, 2019 at 6:25

Julien Kieffer · Accepted Answer · 2019-11-19 06:59:24Z

1

A smart way of doing this is querying User instead of Session. Read more here.

from django.db.models import Q
# ... Other import here

users = User.objects.all(Q(leader__isnull=False) | Q(follower__isnull=False),).distinct()

This will give you any user where either related relations leader/follower is null.

If you want to add some filters based on sessions values, you could do something like:

sessions = Session.objects.filter(**your_filters)
users = User.objects.all(
    Q(leader__in=sessions) | Q(follower__in=sessions),).distinct()

edited Nov 19, 2019 at 6:59

answered Nov 19, 2019 at 6:21

Julien Kieffer

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4 Comments

sunw Over a year ago

The problem here is that leader and follower are fields of Session, not User.

Julien Kieffer Over a year ago

Using related_name will make your relations available within user instances docs.djangoproject.com/en/2.2/ref/models/fields/…

sunw Over a year ago

Thank you! Finally, let's say that it's not all_sessions = Session.objects.all(), but instead some_sessions = Session.objects.filter(some_criteria=True). (I plan to vary the Session queryset.) How can I get a count of unique Users that have been either a leader or follower in some_sessions?

Julien Kieffer Over a year ago

Just added an example in response

Bhoomi Patel · Accepted Answer · 2019-11-19 06:42:23Z

0

For get a simple count of unique User use count method.

active_user_ids = set()
all_sessions = Session.objects.all()
for session in all_sessions:
    active_user_ids.add(session.leader.id)
    active_user_ids.add(session.follower.id)
all_active_users = User.objects.filter(id__in=active_user_ids).count()

answered Nov 19, 2019 at 6:42

Bhoomi Patel

77510 silver badges32 bronze badges

1 Comment

sunw Over a year ago

Thanks for seeing the typo. It so happens that I already did have that in my code; I just copy/pasted incorrectly. I have updated my question.

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