Expense of django query

Question

I have two tables: Actor and Images. The Images table is linked to the Actor table through a Foreign Key. I might make a query on Actor that pulls a list of all the actors with first name beginning with the letter A. Let's say I store this query result in actor_list and pass it to the template. I can get a list of all the Images through the template by iterating. Something like:

{% for actor in actor_list %}
    {% for image in actor.img_set %}
       {{ image }}
    {% endfor %}
{% endfor %}

Is this costly to the database? Does the associated img_set come through with the initial actor object list, or does a new query hit the database every time I call for the actor.img_set?

Answer 1

You should install the Django debug toolbar and see what SQL queries are happening under the covers.

If you are getting too many queries, you can try using select_related() to pull in the related data in the initial query.

Answer 2

Better than using the django toolbar is using the connections library. With Debug set to True:

>>> from django.db import connection
>>> for actor in actor_list:
>>>  for image in actor.img_set:
>>>   image
>>> connection.queries()

This will show all the queries that were executed and the amount of time that each took.

The debug toolbar is cool, though, for many uses. :-)