# Does Rails cache join table definitions?

**URL:** <https://www.ruby-forum.com/t/does-rails-cache-join-table-definitions/81748>\
**Category:** Rails\
**Created:** [January 12, 2007, 5:56pm UTC](https://www.ruby-forum.com/t/does-rails-cache-join-table-definitions/81748 "2007-01-12T17:56:27Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![farfignugen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/f17d59/32.png) [@farfignugen](https://www.ruby-forum.com/u/farfignugen)\
**Post date:** [January 12, 2007, 5:56pm UTC](https://www.ruby-forum.com/t/does-rails-cache-join-table-definitions/81748/1 "2007-01-12T17:56:27Z")

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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)

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<div class="post-metadata">

**Author:** ![farfignugen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/f17d59/32.png) [@farfignugen](https://www.ruby-forum.com/u/farfignugen)\
**Post date:** [January 13, 2007, 7:06am UTC](https://www.ruby-forum.com/t/does-rails-cache-join-table-definitions/81748/2 "2007-01-13T07:06:51Z")

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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).