ActiveRecord Associations issue

rubyonrails-talk
1

Hello all and thank you for reading this message.

I am new to RoR, and ruby too, but i am quite excited about it. I am developing a RoR project and with active record associations in the following

manner

# Tables (all tables have only one record) CREATE TABLE `webcars_development`.`cars` (   `id` int(11) NOT NULL auto_increment,   `model_id` int(11) NOT NULL default '0',   `photo` blob,   `created_at` datetime default NULL,   `updated_at` datetime default NULL,   PRIMARY KEY (`id`),   KEY `FK_CARS_MODELS` (`model_id`),   CONSTRAINT `FK_CARS_MODELS` FOREIGN KEY (`model_id`) REFERENCES `models` (`id`) );

CREATE TABLE `webcars_development`.`models` (   `id` int(11) NOT NULL auto_increment,   `name` varchar(64) NOT NULL default '',   `brand_id` int(11) NOT NULL default '0',   `created_at` datetime default NULL,   `updated_at` datetime default NULL,   PRIMARY KEY (`id`),   UNIQUE KEY `name_idx` (`nome`),   KEY `FK_MODELS_BRAND` (`brand_id`),   CONSTRAINT `FK_MODELS_BRAND` FOREIGN KEY (`brand_id`) REFERENCES `brands` (`id`) );

CREATE TABLE `webcars_development`.`brands` (   `id` int(11) NOT NULL auto_increment,   `name` varchar(16) NOT NULL default '',   `created_at` datetime default NULL,   `updated_at` datetime default NULL,   PRIMARY KEY (`id`),   UNIQUE KEY `name_idx` (`name`) );

# Models class Car < ActiveRecord::Base   belongs_to :model   has_one :brand, :through => :models

  validates_presence_of :model_id end

class Model < ActiveRecord::Base   belongs_to :brand   has_one :car, :through => :models

  validates_associated :brand   validates_presence_of :brand, :message => 'is unknown' end

class Marca < ActiveRecord::Base   has_one :model, :through => :brands end

# Controller (only index action needed for the example) class CarsController < ApplicationController   def index     @cars = Cars.find(:all)   end end

# The view <table>   <tr>     <th>Brand</th>     <th>Model</th>     <th>Photo</th>   </tr> <% for car in @cars %>   <tr>     <td><%=h car.model.brand.name %></td>     <td><%=h car.model.name %></td>     <td><%=h car.photo %></td>   </tr> <% end %>

Ok, so my question is this. When i went trough the MySQL logs i saw that active record calls each table on its time instead of joining them. It runs the following queries:

SELECT * FROM `cars` SHOW FIELDS FROM `cars` SHOW FIELDS FROM `models` SELECT * FROM `models` WHERE (`models`.`id` = 1) SHOW FIELDS FROM `brands` SELECT * FROM `brands` WHERE (`brands`.`id` = 1)

should it not be running a query similar to this one?

SHOW FIELDS FROM `cars` SHOW FIELDS FROM `models` SHOW FIELDS FROM `brands` SELECT

2

True, but it would be excessive to run 1 query with 3 joins if you weren't going to access the associations at all (and would be even worse if you had lots of associations). Have a read through the api docs on eager-loading (the :include option).

Fred

3

Thank you for your answer, i will look in to eager-loading.

But please clarify one thing for me. You said that its excessive to run the joined query if im not going to access the associations, but in the views when i type "cars.model.brand" isn't that accessing the AR associations? (it's a simple example i know)

Thanks again for the help

4

Thank you for your answer, i will look in to eager-loading.

But please clarify one thing for me. You said that its excessive to run the joined query if im not going to access the associations, but in the views when i type "cars.model.brand" isn't that accessing the AR associations? (it's a simple example i know)

Yes you are. My point was that at the point at which you do find :all, rails doesn't know that you're going to do that.

Fred