Retrive Django related table data based on entity type and id

Question

I am having some difficulties to write this django queries. I have the following scenario.

I have three django models

• User
• Visitor
• Participants

For each interaction of a user and a visitor, I add some participants

participant_id entity_type entity_id 
    1             user        1
    2             visitor     1
    3             user        1
    4             visitor     2
    5             user        1
    6             visitor     3

Entity type can be either user or visitor. How can I get related user and visitor data from entity id and entity type?

Answer 1

If you can refactor your solution to generic-relations, the answer is easy, because django already implements solution for you:

class Participant(Model):

    entity_type = ForeignKey(ContentType)
    entity_id = CharField(_('Sourced object id'))

    content_object = GenericFields('entity_type', 'entity_id')

And after:

user_or_visitor = participant.content_object 

more here: https://docs.djangoproject.com/en/5.0/ref/contrib/contenttypes/#generic-relations

Don't forget that you are getting here the “N+1 queries” performance problem "by default" and should take care of it with GenericPrefetch(). more here: https://docs.djangoproject.com/en/5.0/ref/contrib/contenttypes/#genericprefetch

If you can not change current project models:

Preparing data:

participants = Participant.objects.all()

visitors_ids = participants.objects.filter(entity_type="visitor").values_list('pk', flat=True)
users_ids = participants.objects.filter(entity_type="user").values_list('pk', flat=True)

users = User.objects.filter(id__in=users_ids).in_bulk()
visitors = Visitors.objects.filter(id__in=visitors_ids).in_bulk()

Here you have made only 2 database queries, but you have stored the objects in memory.

Somewhere later:

for participant in participants.objects.all():
    participant.entity = None
    if participant.entity_type == "visitor":
        participant.entity = visitors[participant.entity_id]
    elif participant.entity_type == "user":
        participant.entity = users[participant.entity_id]

Here you have made only one request to db.

Of course, I can offer a more complex queryset request solution, but in my opinion, simple is better than complex.