Django JOIN query without foreign key

Question

Is there a way in Django to write a query using the ORM, not raw SQL that allows you to JOIN on another table without there being a foreign key? Looking through the documentatio

Answer 1

Just reposting some related answer, so everyone could see it. Taken from here: Most efficient way to use the django ORM when comparing elements from two lists

First problem: joining unrelated models

I'm assuming that your Model1 and Model2 are not related, otherwise you'd be able to use Django's related objects interface. Here are two approaches you could take:

1. Use extra and a SQL subquery:

   Model1.objects.extra(where = ['field in (SELECT field from myapp_model2 WHERE ...)'])
   

   Subqueries are not handled very efficiently in some databases (notably MySQL) so this is probably not as good as #2 below.

2. Use a raw SQL query:

   Model1.objects.raw('''SELECT * from myapp_model1
                      INNER JOIN myapp_model2
                      ON myapp_model1.field = myapp_model2.field
                      AND ...''')
   

Second problem: enumerating the result

Two approaches:

1. You can enumerate a query set in Python using the built-in enumerate function:

   enumerate(Model1.objects.all())
   

2. You can use the technique described in this answer to do the enumeration in MySQL. Something like this:

   Model1.objects.raw('''SELECT *, @row := @row + 1 AS row
                      FROM myapp_model1
                      JOIN (SELECT @row := 0) rowtable
                      INNER JOIN myapp_model2
                      ON myapp_model1.field = myapp_model2.field
                      AND ...''')