在Django中查询相关集合

Question

I'm working on a Django app that has the following layout (trimmed down and anonymized):

class Room(models.Model):
    pass # Bunch of fields here

class Reserveration(models.Model):
    room = models.ForeignKey(Room)
    start_date = models.DateTimeField()
    end_date = models.DateTimeField()
    # more stuff

Now I'd like to do some queries that relate rooms and their collection of reservations. (I guess most/all of what I want to do involves selecting into a temp table...) Following are examples of the stuff I'd like to do (written in LINQ). I understand I could do these in a for loop, but I'd like to keep this in the database.

// Return all rooms that are reserved in the future
return Rooms.Where(r => Reservations.Where(a => a.room == r)
       .OrderByDecending(a => a.end_date)
       .Single()
       .end_date > DateTime.Now);

// Return all rooms that are free right now
return Rooms.Where(r => ! Reservations.Any(a => a.room == r &&
            a.start_date < DateTime.Now && DateTime.Now < a.end_date));

I can't seem to find any information about how to preform these types of queries in Django without resorting to raw SQL. Is what I'm trying even possible?

Answer 1

From https://docs.djangoproject.com/en/dev/topics/db/queries/ :

Use r.room_set to get the relevant queryset.

Your first query translates to:

r.room_set.filter(end_date__gt=datetime.date.today()).order_by('end_date')[:1]

This returns an empty set if there is no match, or a set with the first element.

The second is:

r.room_set.filter(start_date__lt=datetime.date.today(),
    end_date__gt=datetime.date.today())