ActiveRecord puzzler for smart kids (preserving fk inserts when using has_many :through, :condition

Rails
1

You kids and your fancy ActiveRecord stuff. You’re having all the fun
while us geezers are writing complex JOINS by hand. Whee!

Wondering if anyone has reached a more elegant solution to this sort
of has_many :through association.

Say I’ve got Jobs, Participations, Roles and Users.

A Job has_many Users through Participations.

A Participation belongs_to a Role, User, and Job.

One type of role is a “Candidate”, represented as a Role object
Role::CANDIDATE.

To create a custom AR collection like job.candidates, I’ve implemented
the following:
(sorry, pastie is broken right now)

class Job
has_many :participations
has_many :users, :through => :participations
has_many :candidates, :through => :participations,
:source => :user,
:conditions => [“role_id = ?”,
Role::CANDIDATE.id]
end

So job.candidates yields:
SELECT “users”.* FROM “users” INNER JOIN participations ON users.id =
participations.user_id WHERE ((“participations”.job_id = 28) AND
((role_id = 6)))

And job.candidates << my_user yields:
INSERT INTO “participations” (“job_id”, “updated_at”, “role_id”,
“user_id”, “created_at”) VALUES(28, ‘2008-07-10 17:52:55.031711’,
NULL, 11, ‘2008-07-10 17:52:55.031711’)

It would be great if AR would, for through relationships with
conditions, automatically take the condition clause and be smart
enough to intelligently supply the id value during the INSERT. In
other words, I need that NULL value to be ‘6’.

My first approach to this problem was to try using a :before_add
function, like so:

:before_add => Proc.new {|job, user| #would love to do it here}

But :before_add only receives a reference to the local and foreign
object, eg the Job and the User. It would be great if somehow I could
get a reference to the object in the join (Participation) such that:

:before_add => Proc.new {|job, participation, user| participation.role
= Role::CANDIDATE}

I realize I can create a method add_candidate(user) and manually do
this stuff… but has anyone encountered a similar situation and
reached an elegant solution?

Thanks for taking the time to read this.

2

Hi –

On Thu, 10 Jul 2008, tshim wrote:

has_many :participations
((role_id = 6)))

And job.candidates << my_user yields:
INSERT INTO “participations” (“job_id”, “updated_at”, “role_id”,
“user_id”, “created_at”) VALUES(28, ‘2008-07-10 17:52:55.031711’,
NULL, 11, ‘2008-07-10 17:52:55.031711’)

It would be great if AR would, for through relationships with
conditions, automatically take the condition clause and be smart
enough to intelligently supply the id value during the INSERT. In
other words, I need that NULL value to be ‘6’.

I don’t think the conditions can be reverse-engineered in the general
case, though. It might be [“role_id in (?)”, [bunch,of,values]], for
example.

= Role::CANDIDATE}

I realize I can create a method add_candidate(user) and manually do
this stuff… but has anyone encountered a similar situation and
reached an elegant solution?

Try this:

has_many :candidates,
:through => :participations,
:source => :user,
:conditions => [“role_id = ?”, Role::CANDIDATE] do
def <<(user)
Participation.create(:user_id => user.id,
:role_id => Role::CANDIDATE,
:job_id => proxy_owner.id)
end
end

It’s the (very cool) technique of adding custom methods to association
collections. This one is inspired by Josh S.'s blog post on this
problem (customizing <<).[1]

David

[1] has_many :through - New on edge: Magic join model creation

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Intro to Ruby on Rails July 21-24 Edison, NJ
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3

Hi David, thanks for contributing to the group and for taking the time
to help me specifically.

That’s exactly the behavior I was trying to implement. I’d seen a
similar post on Josh S.'s hasmanythrough blog… but not the one
you pointed out.

You get a gold star!

Readers of this thread should totally explore this! It’s a transparent
but powerful approach to controlling your AR associations.