# ActiveRecord Associations issue

**URL:** <https://www.ruby-forum.com/t/activerecord-associations-issue/143756>\
**Category:** Rails\
**Created:** [July 22, 2008, 2:06pm UTC](https://www.ruby-forum.com/t/activerecord-associations-issue/143756 "2008-07-22T14:06:30Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![CrazyMenConnected](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/ec9cab/32.png) [@CrazyMenConnected](https://www.ruby-forum.com/u/CrazyMenConnected)\
**Post date:** [July 22, 2008, 2:06pm UTC](https://www.ruby-forum.com/t/activerecord-associations-issue/143756/1 "2008-07-22T14:06:30Z")

</div>

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
\<% for car in @cars %\> \<% 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  
\*  
FROM  
`cars`  
INNER JOIN `models` ON  
`models`.`id` = `cars`.`model_id` AND  
`models`.`id` = 1  
INNER JOIN `brands` ON  
`brands`.`id` = `models`.`brand_id` AND  
`brands`.`id` = 1

What am i doing wrong? Is it the tables, the associations? Or does  
active record simply does not behave like this.  
It seems a little excessive to execute 3 queries when it could all be  
done just in one query with joins.

I am still new to RoR, but this would make since would it not? If you  
have a heavily loaded server, and with a lot of associations this  
would probaly slowdown the overall performance because of too many  
call to the SQL engine.

Any help on this matter would be very great, and i would be very  
thankful!

Thank for the help!

CrazyMenConnected

| Brand | Model | Photo |
| --- | --- | --- |
| \<%=h car.model.brand.name %\> | \<%=h car.model.name %\> | \<%=h car.photo %\> |

---

<div class="post-metadata">

**Author:** ![CrazyMenConnected](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/ec9cab/32.png) [@CrazyMenConnected](https://www.ruby-forum.com/u/CrazyMenConnected)\
**Post date:** [July 22, 2008, 2:08pm UTC](https://www.ruby-forum.com/t/activerecord-associations-issue/143756/2 "2008-07-22T14:08:16Z")

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On Jul 22, 12:34 pm, CrazyMenConnected [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> INNER JOIN `brands` ON  
> `brands`.`id` = `models`.`brand_id` AND  
> `brands`.`id` = 1
> 
> What am i doing wrong? Is it the tables, the associations? Or does  
> active record simply does not behave like this.  
> It seems a little excessive to execute 3 queries when it could all be  
> done just in one query with joins.

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

---

<div class="post-metadata">

**Author:** ![CrazyMenConnected](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/ec9cab/32.png) [@CrazyMenConnected](https://www.ruby-forum.com/u/CrazyMenConnected)\
**Post date:** [July 22, 2008, 3:36pm UTC](https://www.ruby-forum.com/t/activerecord-associations-issue/143756/3 "2008-07-22T15:36:27Z")

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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

On Jul 22, 1:07 pm, Frederick C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)

---

<div class="post-metadata">

**Author:** ![CrazyMenConnected](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/ec9cab/32.png) [@CrazyMenConnected](https://www.ruby-forum.com/u/CrazyMenConnected)\
**Post date:** [July 22, 2008, 11:40pm UTC](https://www.ruby-forum.com/t/activerecord-associations-issue/143756/4 "2008-07-22T23:40:25Z")

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On Jul 22, 2:35 pm, CrazyMenConnected [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> 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