Question

I’m trying to get a set of objects in Django Minus its existence in another set of objects. I have a Message model with a thread field that designates which thread it belongs to and a MessagesRead model that stores the user and the message that he/she has seen. I’m trying to return the messages that he/she has NOT read (to alerts for new messages), i.e. those messages that are not stored as a pair in the MessagesRead table.

Something like the following:

def unread_messages(user, thread_id):
    Message.objects.filter(
        self not in MessagesRead.objects.filter(
            message__thread_id=thread_id,
            user=user,
        )
    )

Can I do the above somehow or is there another way?

Answer 1

this assumes that MessagesRead has a foreign key back to thread with related_name set to messagesread_set

def unread_messages(user, thread_id):
    Message.objects.filter(
            thread_id = thread_id,
        ).exclude(
            messagesread_set__in = MessagesRead.objects.filter(
                message__thread_id = thread_id,
                user = user,
            )
        )