[Django]-Django – filtering on related objects

by

20👍

✅

The filter method is for filtering which objects are returned based on the specified criteria, so it’s not what you want here. One option is to do a second query to retrieve all ratings for given Event objects for the current User.

Models:

import collections

from django.db import models

class RatingManager(models.Manager):
    def get_for_user(self, events, user):
        ratings = self.filter(event__in=[event.id for event in events],
                              user=user)
        rating_dict = collections.defaultdict(lambda: None)
        for rating in ratings:
            rating_dict[rating.event_id] = rating
        return rating_dict

class Rating(models.Model):
    # ...
    objects = RatingManager()

View:

events = Event.objects.all()
user_ratings = Rating.objects.get_for_user(events, request.user)
context = {
    'events': [(event, user_ratings[event.id]) for event in events],
}

Template:

{% for event, user_rating in events %}
  {% if user_rating %} ... {% endif %}
{% endfor %}
👤Jonny Buchanan

6👍

In addition to S.Lott’s suggestion, you may consider using select_related() to limit the number of database queries; otherwise your template will do a query on each event’s pass through the loop.

Event.objects.all().select_related(depth=1)

The depth parameter is not required, but if your other models have additional foreign keys it will limit the number of joins.

👤Daniel Naab

6👍

The best practice would be

from django.db.models import Prefetch


event_list = Event.objects.all().prefetch_related(Prefetch(<related_name>, queryset=Rating.objects.filter(<criteria>)))

<related_name> = ‘ratings’ if:

class Rating(models.Model):
    event = models.ForeignKey(Event, related_name='ratings')

This returns events and filtered ratings for those event objects

👤alper

1👍

To make best use of Django, you have to avoid trying to do joins.

A “left outer join” is actually a list of objects with optional relationships.

It’s simply a list of Events, Event.objects.all(). Some Event objects have a rating, some don’t.

You get the list of Events in your view. You handle the optional relationships in your template.

{% for e in event_list %}
    {{ e }}
    {% if e.rating_set.all %}{{ e.rating_set }}{% endif %}
{% endfor %}

is a jumping-off point.

👤S.Lott

0👍

I think you have to do something like this.

events=Event.objects.filter(rating__user=request.user.id)
ratings='(select rating from ratings where user_id=%d and event_id=event_events.id '%request.user.id
events=events.extra(select={'rating':ratings})
👤fastmultiplication

Leave a comment