How to query from multiple models in django

Question

I am trying to understand the most efficient way to query the data needed from multiple tables in Django.

class City():
    name = models.CharlField()

class Address():
    line1 = models.CharlField()
    ...
    city_id = models.ForeignKey(City, ... related_name="city_address")

class Student()
   name = models.CharlField()
   gender = models.CharlField()
   # a student can only have one address
   address_id = models.OneToOneField(Address, ... related_name="address_student")

How can I optimize the query below to get the city this student belongs to?

student = Student.objects.filter(name="John").first() # assuming its a unique name
address_id = student.address_id
address = Address.objects.filter(id=address).first()
city_id = address.city_id

City.objects.get(id=city_id)

You should be able to get it as student.address.

Gram
– Gram

2021-10-27 11:56:33 +00:00
Commented Oct 27, 2021 at 11:56 — Gram
– Gram, Commented Oct 27, 2021 at 11:56

willeM_ Van Onsem · Accepted Answer · 2021-10-27 11:55:53Z

2

You can query with:

City.objects.get(address__student=mystudent)

This will produce a query that looks like:

SELECT city.*
FROM city
INNER JOIN address ON address.city_id_id = city.id
INNER JOIN student ON student.address_id_id = address.id
WHERE student.id = id_of_the_student

answered Oct 27, 2021 at 11:55

willeM_ Van Onsem

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