[FIXED] Get foreign key objects in a single query

Issue

I have 2 models in my Django code:

class ModelA(models.Model):
    name = models.CharField(max_length=255)
    description = models.CharField(max_length=255)
    created_by = models.ForeignKey(User)

class ModelB(models.Model):
    category = models.CharField(max_length=255)
    modela_link = models.ForeignKey(ModelA, 'modelb_link')
    functions = models.CharField(max_length=255)
    created_by = models.ForeignKey(User)

Say ModelA has 100 records, all of which may or may not have links to ModelB

Now say I want to get a list of every ModelA record along with the data from ModelB

I would do:

list_a = ModelA.objects.all()

Then to get the data for ModelB I would have to do

for i in list_a:
    i.additional_data = i.modelb_link.all()

However, this runs a query on every instance of i. Thus making 101 queries to run.

Is there any way of running this all in just 1 query? Or at least less than the 101 queries.

I've tried putting in ModelA.objects.select_related().all() but this didn't seem to have any effect.

Solution

As Ofri says, select_related only works on forwards relations, not reverse ones.

There's no built-in way to automatically follow reverse relations in Django, but see my blog post for a technique to do it reasonably efficiently. The basic idea is to get all the related objects for every item at once, then associate them manually with their related item - so you can do it in 2 queries rather than n+1.

Answered By - Daniel Roseman