Rails - May 2006 - Validating Foreign Key

Howdy,

I''m a Rails beginner working on my first significant project with RoR. 
I''m trying to figure out how to validate a foreign key (you know, make 
sure the row actually exists.)

However, validates_associated doesn''t seem to be what I want to use,
nor
does it work...

-- BEGIN --
class City < ActiveRecord::Base
   has_many :schools
   belongs_to :state

   validates_presence_of :city_name
   validates_presence_of :state_id
   validates_associated :state
end

class State < ActiveRecord::Base
   has_many :cities

   validates_presence_of :long_name
   validates_presence_of :state_code
   validates_length_of :state_code, :is => 2
   validates_uniqueness_of :state_code
end
-- END --

Regardless of all of that, my test can still create a city with a 
state_id of 91, and I do not have a 91 row in my states table.

-- From test.log --
State Load (0.000907) SELECT * FROM states WHERE (states.id = 91) LIMIT 1
SQL (0.000487) INSERT INTO cities (`city_name`, `state_id`) 
VALUES(''Hello!'', 91)
-- END --

What am I looking to do here...? I googled, but there doesn''t seem to
be
a really Rails-y way to do this. Do I need to write a custom validation...?

Thank you!

- Nick Evans

On May 28, 2006, at 3:37 pm, Nicholas Evans wrote:
> I''m a Rails beginner working on my first significant project with
> RoR. I''m trying to figure out how to validate a foreign key (you  
> know, make sure the row actually exists.)

Nicholas

You need to read the documentation for whatever database server you  
are using.  It will tell you how to make foreign key constraints so  
that you *can''t* create rows in one table that reference non-existant  
rows in another.

This is by far the best way to solve the problem as it is what  
databases are made for.

Ashley

Ashley,

Rails doesn''t work well with foreign key constraints at the database 
level (fixtures can get all messed up because it doesn''t know how to 
load them in the correct order). Plus, I had always been told to never, 
ever, evAr use key constraints and Rails...is that not true?

However, it looks like I''ve figured out my solution. I added a custom 
validate method that tries to find the state.

class City < ActiveRecord::Base
   has_many :schools
   belongs_to :state

   validates_presence_of :city_name
   validates_presence_of :state_id

   # Validate is called before save
   def validate
     unless State.find_by_id(state_id)
       errors.add(:state_id, ''does not exist'')
     end
   end # validate
end
---
19 tests, 30 assertions, 0 failures, 0 errors (win!)

Thank you all the same!

- Nick Evans

Ashley Moran wrote:
> 
> On May 28, 2006, at 3:37 pm, Nicholas Evans wrote:
> 
>> I''m a Rails beginner working on my first significant project
with
>> RoR. I''m trying to figure out how to validate a foreign key
(you
>> know, make sure the row actually exists.)
> 
> 
> 
> Nicholas
> 
> You need to read the documentation for whatever database server you  are 
> using.  It will tell you how to make foreign key constraints so  that 
> you *can''t* create rows in one table that reference non-existant 
rows
> in another.
> 
> This is by far the best way to solve the problem as it is what  
> databases are made for.
> 
> Ashley
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

Nicholas Evans wrote the following on 28.05.2006 16:51 :
> Ashley,
>
> Rails doesn''t work well with foreign key constraints at the
database
> level (fixtures can get all messed up because it doesn''t know how
to
> load them in the correct order). Plus, I had always been told to
> never, ever, evAr use key constraints and Rails...is that not true?
>
> However, it looks like I''ve figured out my solution. I added a
custom
> validate method that tries to find the state.
>
> class City < ActiveRecord::Base
>   has_many :schools
>   belongs_to :state
>
>   validates_presence_of :city_name
>   validates_presence_of :state_id
What you want is:
validates_associated :state

validates_presence_of only tests if self.state_id != nil
(by the way it didn''t exactly do that in Rails 1.0.0, instead it tested
that self.state_id was true which bite me hard for booleans... which can
be false, didn''t test with 1.1.2 yet).

But pay attention to the fact that validates_associated probably isn''t
as robust as real foreign keys (you can delete an object referenced by
another without foreign keys, especially if you have concurrent accesses
which make application-level checks subject to race conditions).

Personally I use both foreign keys and validates_associated.
Foreign keys ensure data consistency, validates_associated in the models
make error handling easier (detecting error types by parsing DBMS error
messages is a pain...)

Note that for fixtures there is a workaround for the current limitations
by loading all inter-dependent fixtures in each test class in the
correct order.

Lionel.