Does Rails cache join table definitions?

Rails
1

I’m developing an application with a simple user
authentication/authorization system (as in Rails Recipes, Recipes 31 and
32). Of course, this requires two join tables (one between users and
groups, and one between groups and permissions).

I set config.cache_classes = true in my environments/development.rb in
order to simulate the behavior of the production environment, and then
took a look at the log. Whenever I access data from a model table
(users, groups, permissions, orders), an SQL call is made to determine
the table structure the first time the table is accessed, and that call
is not made again. (An example is below.)

SQL (0.005302) SELECT a.attname, format_type(a.atttypid,
a.atttypmod), d.adsrc, a.attnotnull
FROM pg_attribute a LEFT JOIN pg_attrdef d
ON a.attrelid = d.adrelid AND a.attnum = d.adnum
WHERE a.attrelid = ‘users’::regclass
AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum

However, SQL calls to determine the table structure of the groups_users
and groups_permissions tables are made every single time I call
user.groups, group.permissions, etc. (Example below.)

SQL (0.001137) SELECT a.attname, format_type(a.atttypid,
a.atttypmod), d.adsrc, a.attnotnull
FROM pg_attribute a LEFT JOIN pg_attrdef d
ON a.attrelid = d.adrelid AND a.attnum = d.adnum
WHERE a.attrelid = ‘groups_users’::regclass
AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum

I’m probably prematurely optimizing, but I wonder how much these
additional SQL calls will affect database performance, both with the
user/group/permission tables (which get called for every action) and
other many-to-many join tables in my application. Has anyone else
noticed this?

Thanks,
Chris

(Ruby 1.8.4, Rails 1.2RC1, MacOS X 10.4.8 Intel, PostgreSQL 8.1.3)

2

I get the same behaviour - I think what’s happening is that the column
information is cached in the corresponding model class. Since join
tables don’t have a corresponding model, there’s nowhere for the
information to be cached.

If you’re really worried about this you can get around it by creating an
explicit model for the join table and using the has_many :through option
(see the API docs for ActiveRecord::Associations::ClassMethods).