Order a queryset by a field in another queryset - django

Question

In my django blog app, I have a model for Bookmark, which includes fields pointing to Post and User, and a field for when the bookmark was created.
# models
class Post(models.Model):
# ...
title = models.CharField(max_length=70)
body = RichTextField()
author = models.ForeignKey(
UserProfile,
on_delete=models.CASCADE,
related_name='posts')
created_on = models.DateTimeField(default=timezone.now)
# ...
class Meta:
ordering = ['created_on']
class Bookmark(models.Model):
post = models.ForeignKey(
Post, on_delete=CASCADE,
related_name="bookmarked_post"
)
user = models.ForeignKey(
User, on_delete=CASCADE,
related_name="bookmarks"
)
created_on = models.DateTimeField(default=timezone.now)
class Meta:
ordering = ['created_on']
I have a view to display my users' bookmarks. I want to order the posts queryset in the order they were bookmarked, with the recently bookmarked posts first.
# views.py
#login_required
def bookmarks_view(request):
bookmarks = Bookmark.objects.filter(user=request.user)
bookmarked_posts = Post.objects.filter(
bookmarked_post__in=bookmarks).order_by(???)
return render(request, 'bookmarks.html', {'posts': bookmarked_posts})
I'm struggling to figure out how to order my Posts queryset by the created_on field in the Bookmark model. To add additional complexity, both the Post and Bookmark model have a created_on field.
Can anyone point me in the right direction with this? I've scoured the django documentation and SO and not finding what I need, possibly I just don't know the right keywords... Or have I set up my models incorrectly to achieve what I want?

Answer 1

Something like this should work:
bookmarks = Bookmark.objects.filter(user=u0)
bookmarked_posts = Post.objects.filter(
bookmarked_post__in=bookmarks
).order_by("-bookmarked_post__created_on")
You can drop the __created_on at the end - Django will then use the default ordering specified on the related model. The - at the beginning means descending, so newer dates come first.
See also the documentation for order_by:
To order by a field in a different model, use the same syntax as when you are querying across model relations. That is, the name of the field, followed by a double underscore (__), followed by the name of the field in the new model, and so on for as many models as you want to join.
Because you specified the related name as bookmarked_post, that's also the name of the field you need to specify, from the perspective of Post, just like you used it in the filter.
Without the related_name, it would look like this:
bookmarks = Bookmark.objects.filter(user=u0)
bookmarked_posts = Post.objects.filter(
bookmark__in=bookmarks
).order_by("-bookmark__created_on")
(Note that you may want to specify a unique_together constraint for post and user on the Bookmark model, to make sure that a post can only be bookmarked once per user)