How can I do this complex SQL query using Django ORM? (sub-query with a join)

Question

I'm used to writing my own SQL queries and I'm trying to get used to the whole ORM thing that seems to be so popular nowadays.

Here's the query:

SELECT * FROM routes WHERE route_id IN (
    SELECT DISTINCT t.route_id FROM stop_times AS st 
    LEFT JOIN trips AS t ON st.trip_id=t.trip_id
    WHERE stop_id = %s
)

where %s is an integer.

I'm using Django's default ORM. What's the most pythonic way to do this?

Some background info: The DB I'm using is from a GTFS (Google Transit feed specification). This query is supposed to get a list of every route that goes through a particular stop , however the info linking these is in the trips table.

This query works just fine for me, so the only reason I'm asking is to learn.

Thanks!

Answer 1

Correct me if I'm wrong, but I don't think you can do that with Django ORM in a normal way.

There is no subquery support and with a normal join it would depend on your database if a distinct could help you. If you are using Postgres than you could do it with this patch: http://code.djangoproject.com/ticket/6422

The query would be something like this:

Route.objects.filter(stop_time__trips__stop_id=...).distinct('stop_time__route_id')

Answer 2

It'd probably be a bit easier to figure out the appropriate way to do this if you had what you were using for the relevant Models .

I'm assuming something like the following, based on the specification you mentioned working from:

class Route(models.Model):
    #bunch of stuff
    pass
class Stop(models.Model):
    #bunch of stuff
    stop_times = models.ManyToManyField(through=StopTime)
class StopTime(models.Model):
    trip = models.ForeignKey(Trip)
    stop = models.ForeignKey(Stop)
    # bunch of additional meta about this M2M table
    pass
class Trip(models.Model):
    route = models.ForeignKey(Route)
    # bunch of stuff

If that's the case... you should be able to do something like

Route.objects.filter(trip__stop__id=my_stop_id)

to get all Route objects that go through a given Stop with a primary key id equal to my_stop_id , which I'm assuming is an integer as per your post.

I apologize if the syntax is a bit off, as I haven't needed to do many-to-many relationships using an explicit extra table. Some adjustment may also be needed if you have to (or choose to) use the related_name parameter for any the foreign keys or the many-to-many-field.