django select_related for multiple foreign keys

Question

How does select_related work with a model which has multiple foreign keys? Does it just choose the first one?

class Model: fkey1, fkey2, fkey3...

The documentation doesn't say anything about this, at least not in where the method is specified.

NOTE: Maybe I'm the only one who will get confused. I guess select_related is just a performance booster (I can see that) but I had the wrong idea that it was something else.

Answer 1

If your model has multiple foreign keys you can:

• Call .select_related(), that will “follow” all non-null foreign-key relationships
• Call .select_related('foreign_key1', 'foreign_key2', ...), that will “follow” only the foreign-key provided as arguments.

Note that "to follow a FK relationship" means selecting additional related-object data when the query is executed (by performing a SQL join). This will make the main query heavier but can be used to avoid N + 1 queries problem.

According to select_related documentation, the first method (without arguments) is not recommended as "it is likely to make the underlying query more complex, and return more data, than is actually needed."

---

If your model has "nested" foreign keys with other models (i.e. Book <>-- Author <>-- Hometown) you can also use select_related as follow:

• Call Book.select_related('author__hometown'), that will “follow” the author's foreign-key (in Book model) and the hometown's foreign-key (in Author model).

---

If your model has many-to-many or many-to-one relations you would like to retrieve from the database, you should take a look at prefetch_related.

Answer 2

You can use select_related in a chain as following

Comment.objects.select_related('user').select_related('article').all()

Answer 3

On the contrary, the documentation is very clear on the matter. It says that by default all ForeignKeys are followed, but you can give the method a list of fields and it will only follow those relationships.