# Depot Application Tutorial and Foreign Keys

**URL:** <https://www.ruby-forum.com/t/depot-application-tutorial-and-foreign-keys/88882>\
**Category:** Rails\
**Created:** [March 15, 2007, 9:30am UTC](https://www.ruby-forum.com/t/depot-application-tutorial-and-foreign-keys/88882 "2007-03-15T09:30:11Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![vagabond](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/258eb7/32.png) [@vagabond](https://www.ruby-forum.com/u/vagabond)\
**Post date:** [March 15, 2007, 9:30am UTC](https://www.ruby-forum.com/t/depot-application-tutorial-and-foreign-keys/88882/1 "2007-03-15T09:30:11Z")

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I’ve been working my way through the depot application tutorial in the  
Agile Rails book, and I’m at the “Check Out!” chapter. In the migration  
file for the line items, there are SQL commands to create foreign keys.  
Up until this point I have been using SQLite3, which as far as I’m aware  
doesn’t support these functions. However, after briefly scanning through  
the rest of the chapter, it seemed to me that these links between  
database tables are handled in the model files, with has\_many and  
belongs\_to, so I ploughed on regardless.

Having got to the end of the chapter with everything working as  
described in the book, I am wondering if there really is any need to set  
up foreign keys in the database. Is it just “good practice” when  
designing database schemas? And does the Ruby code handle this  
functionality satisfactorily without foreign keys?

Please excuse me for my ignorance regarding databases - I’m not a web  
developer by trade, I’m a Structural Engineer - but I’d like to gain a  
greater understanding of the core principals concerning Rails.

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**Author:** ![vagabond](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/258eb7/32.png) [@vagabond](https://www.ruby-forum.com/u/vagabond)\
**Post date:** [March 15, 2007, 3:31pm UTC](https://www.ruby-forum.com/t/depot-application-tutorial-and-foreign-keys/88882/2 "2007-03-15T15:31:29Z")

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Your associations, validations, and callbacks emulate foreign keys so  
there’s no reason to define them in terms of data constraints.

class Foo \< ActiveRecord::Base  
belongs\_to :bar  
validates\_presence\_of :foo\_id  
validates\_associated :foo  
end

class Bar \< ActiveRecord::Base  
has\_many :foos, :dependent =\> :destroy  
end

You can add indexes via a migration.

However, one thing that foreign keys give you is an index on that  
column.  
So if you don’t declare them as foreign keys (which I don’t) then you  
should  
declare an index on those columns to help with lookups.

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<div class="post-metadata">

**Author:** ![vagabond](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/258eb7/32.png) [@vagabond](https://www.ruby-forum.com/u/vagabond)\
**Post date:** [March 15, 2007, 4:41pm UTC](https://www.ruby-forum.com/t/depot-application-tutorial-and-foreign-keys/88882/3 "2007-03-15T16:41:58Z")

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Brian H. wrote:

> Your associations, validations, and callbacks emulate foreign keys so  
> there’s no reason to define them in terms of data constraints.

[snip]

> However, one thing that foreign keys give you is an index on that  
> column.  
> So if you don’t declare them as foreign keys (which I don’t) then you  
> should  
> declare an index on those columns to help with lookups.

Thank you for the explanation.

Is there a situation where creating foreign keys, as opposed to indices,  
would be required? Say, if you wanted the database to be used by other  
(non-Rails) applications or administration tools?

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<div class="post-metadata">

**Author:** ![vagabond](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/258eb7/32.png) [@vagabond](https://www.ruby-forum.com/u/vagabond)\
**Post date:** [March 15, 2007, 6:50pm UTC](https://www.ruby-forum.com/t/depot-application-tutorial-and-foreign-keys/88882/4 "2007-03-15T18:50:05Z")

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Of course. But you get into that sticky situation of moving business  
logic  
into your database. This is one reason SOA is becoming so popular…  
make  
application A use application B to control the database rather than  
going to  
it directly.

You’ll touch off a holy war here if you’re not careful 🙂 The main  
point is  
that Rails does not need nor require them, and you may get unexpected  
results if, for example, your database has constraints of the foreign  
key  
that restricts its input but you have not handled that in the models, or  
perhaps you cascade-delete records on your database and you don’t deal  
with  
that in your apps.

In short, it’s all up to you. There are ways to handle all of those  
concerns; if you need to use FKs, by all means do so.

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<div class="post-metadata">

**Author:** ![vagabond](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/258eb7/32.png) [@vagabond](https://www.ruby-forum.com/u/vagabond)\
**Post date:** [March 15, 2007, 9:49pm UTC](https://www.ruby-forum.com/t/depot-application-tutorial-and-foreign-keys/88882/5 "2007-03-15T21:49:30Z")

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Brian H. wrote:

> In short, it’s all up to you. There are ways to handle all of those  
> concerns; if you need to use FKs, by all means do so.

I’m happy to steer clear of them, seeing as I don’t really know anything  
about them 😉 I was just curious as to whether a Rails app would  
benefit from them or not.

Thanks again for your explanation.