Django - join between different models without foreign key - sql

Question

Imagine I have two simple models (it's not really what I have but this will do):
Class Person(models.Model):
person_id = models.TextField()
name = models.TextField()
#...some other fields
Class Pet(models.Model):
person_id = models.TextField()
pet_name = models.TextField()
species = models.TextField()
#...even more fields
Here's the key difference between this example and some other questions I read about: my models don't enforce a foreign key, so I can't use select_related()
I need to create a view that shows a join between two querysets in each one. So, let's imagine I want a view with all owners named John with a dog.
# a first filter
person_query = Person.objects.filter(name__startswith="John")
# a second filter
pet_query = Pet.objects.filter(species="Dog")
# the sum of the two
magic_join_that_i_cant_find_and_possibly_doesnt_exist = join(person_query.person_id, pet_query.person_id)
Now, can I join those two very very simple querysets with any function?
Or should I use raw?
SELECT p.person_id, p.name, a.pet_name, a.species
FROM person p
LEFT JOIN pet a ON
p.person_id = a.person_id AND
a.species = 'Dog' AND
p.name LIKE 'John%'
Is this query ok? Damn, I'm not sure anymore... that's my issue with queries. Everything is all at once. But consecutive queries seem so simple...
If I reference in my model class a "foreign key" (for select_related() use), will it be enforced in the database after the migration? (I need that it DOESN'T happen)

Answer 1

Make a models.ForeignKey but use db_constraint=False.
See https://docs.djangoproject.com/en/3.0/ref/models/fields/#django.db.models.ForeignKey.db_constraint
Also, if this model is managed=False, ie it is a legacy db table and you're not using Django migrations, the constraint won't ever be made in the first place and it's fine.

Answer 2

If you create a FK in the model, Django will create a constraint on migration, so you want to avoid that in your case.
I don't think there is a way to join in the database in Django if you don't declare the field to join as a foreign key. The only thing you can do is to do the join in Python, which might or might not be OK. Think that prefetch_related does precisely this.
The code would be something like:
person_query = Person.objects.filter(name__startswith="John")
person_ids = [person.id for person in person_query]
pet_query = Pet.objects.filter(species="Dog", person_id__in=person_ids).order_by('person_id')
pets_by_person_id = {person_id: pet_group for person_id, pet_group in itertools.groupby(pet_query, lambda pet: pet.person_id)}
# Now everytime you need the pets for a certain person
pets_by_person_id(person.id)
# You can also set it in all objects for easy retrieval
for person in person_query:
person.pets = pets_by_person_id(person.id)
The code might not be 100% accurate, but you get the idea I hope.