Django：按元素属性过滤查询集

Question

I have two models, Chat and DeletedChat. Now i want to get all chats of a user , removing those which from_date -fields, in DeletedChat , are bigger then the last_updated ones of the chat itself. How do i do that?

class Chat(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    last_updated = models.DateTimeField(auto_now=True)

class DeletedChat(models.Model):
    chat = models.ForeignKey(Chat, on_delete=models.CASCADE)
    from_date = models.DateTimeField(default=timezone.now)

And in my views.py I tried:

chat_ids = request.user.deletedchat_set.exclude(from_date__lt='chat__last_updated').values_list('chat', flat=True)

which gives me the following error: ['“chat__message__set__last” value has an invalid format. It must be in YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ] format.'] ['“chat__message__set__last” value has an invalid format. It must be in YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ] format.']

Thanks for your Help!

Answer 1

You can work with an F object to refer to a field:

from django.db.models import F, Q

Chat.objects.filter(Q(deletedchat=None) | Q(deletedchat__form_date__lt=F('last_updated')))

This will return Chat objects for which there is no related DeletedChat object, or where the DeletedChat object has a form_date that is less than the last_updated field of the Chat object.

We can also make use of .exclude() :

from django.db.models import F

Chat.objects.exclude()

The two are however not equivalent if there are multiple related DeleteChat objects. In that case the .filter(..) variant can still contain a Chat if there exists at least one related DeleteChat object for which the from_date is less than the last_updated field. For the .exclude() , it will exclude it from the moment one of the from_date s is greater than the last_updated field.

In case a Chat object has at most one DeletedChat object, you might want to conider a OneToOneField [Django-doc] instead.