Django: Ordered list of model instances from different models?

Question

Users can upload three different types of content onto our site: image, video, audio. Here are the models for each type:

class ImageItem(models.Model):
    user = models.ForeignKey(User)
    upload_date = models.DateTimeField(auto_now_add=True)
    image = models.ImageField(upload_to=img_get_file_path)
    title = models.CharFiled(max_length=1000,
                             blank=True)

class VideoItem(models.Model):
    user = models.ForeignKey(User)
    upload_date = models.DateTimeField(auto_now_add=True)
    video = models.FileField(upload_to=vid_get_file_path)
    title = models.CharFiled(max_length=1000,
                             blank=True)

class AudioItem(models.Model):
    user = models.ForeignKey(User)
    upload_date = models.DateTimeField(auto_now_add=True)
    audio = models.FileField(upload_to=aud_get_file_path)
    title = models.CharFiled(max_length=1000,
                             blank=True)

I have a page called library.html, which renders all the items that a user has uploaded, in order from most recently uploaded to oldest uploads (it displays the title and upload_date of each instance, and puts a little icon on the left symbolizing what kind of item it is).

Assuming it requires three separate queries, how can I merge the three querysets? How can I make sure they are in order from most recently uploaded?

[see here][1] Take a look at That answer :) Ordering it shouldn't be too hard. [1]: http://stackoverflow.com/questions/431628/how-to-combine-2-or-more-querysets-in-a-django-view%20Take%20a%20look%20at%20this%20answer%20:%29 — Frantz Romain, Feb 01 '13 at 03:14

acjay · Answer 1 · 2013-07-01T04:13:37.833

As an alternative, you can use multi-table inheritance and factor common attributes into a superclass model. Then you just order_by upload date on the superclass model. The third-party app django-model-utils provides a custom manager called Inheritance manager that lets you automatically downcast to the subclass models in your query.

from model_utils.managers import InheritanceManager

class MediaItem(models.Model):
    objects = InheritanceManager()

    user = models.ForeignKey(User)
    upload_date = models.DateTimeField(auto_now_add=True)
    title = models.CharFiled(max_length=1000,
                             blank=True)

class ImageItem(MediaItem):
    image = models.ImageField(upload_to=img_get_file_path)

class VideoItem(MediaItem):
    video = models.FileField(upload_to=vid_get_file_path)

class AudioItem(MediaItem):
    audio = models.FileField(upload_to=aud_get_file_path)

Then, your query is just:

MediaItem.objects.all().order_by('upload_date').select_subclasses()

This way, you get what you want with one just query (with 3 joins). Plus, your data is better normalized, and it's fairly simple to support all sorts more complicated queries, as well as pagination.

Even if you don't go for that, I'd still use abstract base class inheritance to conceptually normalize your data model, even though you don't get the database-side and ORM benefits.

This is definitely the way to do it. It's much easier to maintain if you need more media types in the future. Also it has the reuse property of good object oriented programming. — CrazyCasta, Jun 27 '13 at 20:26
Shoutout to Nasmon for catching a typo. It appears that his attempted edit got rejected by third-party mods, but I wanted to give credit where it was due, since I couldn't figure out how to override and accept the edit. — acjay, Jul 01 '13 at 04:14

score 0 · Answer 2 · answered Feb 01 '13 at 07:06

attrgetter can be used to pull out attributes of objects that you may want to key a sort by.

from operator import attrgetter

results = []
results.extend(list(AudioItem.objects.filter(...)))
results.extend(list(VideoItem.objects.filter(...)))
results.extend(list(ImageItem.objects.filter(...))
results.sort(key=attrgetter("upload_date")

score 0 · Accepted Answer · answered Feb 02 '13 at 03:21

Referencing the question, this is the exact solution I used. Influenced by monkut's answer and the top answer from Frantzdy's link in his comment (thanks guys).

from itertools import chain
from operator import attrgetter

images = ImageItem.objects.filter(user=user)
video = VideoItem.objects.filter(user=user)
audio = AudioItem.objects.filter(user=user)
result_list = sorted(chain(images, video, audio),
                     key=attrgetter('upload_date'),
                     reverse=True)

Django: Ordered list of model instances from different models?

3 Answers3