Django formset , queries for relational field for every form

You are fine. This code will cost you only 3 queries.As you can see in select_related() documentation:

Returns a QuerySet that will “follow” foreign-key relationships, selecting additional related-object data when it executes its query. This is a performance booster which results in a single more complex query but means later use of foreign-key relationships won’t require database queries.

It means that your code will preform a mysql join and will result a dataset with all of the data. So by that, I can see that your code is pretty good.

I suggest you use some kind of profiling like django-silk to see how many queries are being generated.

Footnote:

As you can see in prefetch_related() documentation, the difference between prefetch_related() and select_related() is in the way they preform the join:

This (prefetch_related) has a similar purpose to select_related, in that both are designed to stop the deluge of database queries that is caused by accessing related objects, but the strategy is quite different.

...

select_related works by creating an SQL join and including the fields of the related object in the SELECT statement. For this reason, select_related gets the related objects in the same database query.

...

prefetch_related, on the other hand, does a separate lookup for each relationship, and does the ‘joining’ in Python.

So as long as you need one-to-one relationship, select_related is the most efficient way to query the relationship.

I recently realized that the problem in a similar case was not as such the formsets, the problem was the {{formset.errors}} in the template, since each formset generated one query for this.