Django ORM: avoid multiple queries when saving models with foreign key relationships

Question

Let's suppose I have 3 models: A, B, C, with C having foreign keys relationship with both A and B. How can I avoid multiple queries, when saving a C model if I have the necessary info to retrieve A and B?

Currently my code is something like:

a = A.objects.get(title='the title', platform='the platform')
b = A.objects.get(id='the id')
C.objects.update_or_create(a=a, b=b, defaults={'c_param_1':'value'})

this sucks, since it executes at least 3 queries (4 if the C object does not exist). I would like to retrieve a and b at the db layer... how can be implemented using Django's ORM?

I currently tried to use Q objects in this way:

C.objects.update_or_create(
        a=Q(a__title='the title') & Q(a__platform='the platform'), 
        b=Q(b__id='the id'), 
        c_param_1='value'
)

But I get:

TypeError: int() argument must be a string or a number, not 'Q'

ps: I'm using Django 1.7

Arseniy · Accepted Answer · 2014-10-03 04:11:13Z

1

Well, since foreign key is actually id of object it's refferring to, you have to run a query to get this id. But if you already have id of parent object you can create a child object w/o fully loading parent:

C.objects.update_or_create(
    a_id=id_of_a, 
    b_id=id_of_b,
    c_param_1='value'
)

edited Oct 3, 2014 at 4:11

answered Oct 2, 2014 at 9:47

Arseniy

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