ActiveRecord puzzler for smart kids (preserving fk inserts when using has_many :through, :conditions, :before_add)

rubyonrails-talk
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 --

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'.

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.

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?

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 Susser's blog post on this problem (customizing <<).[1]

David

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

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 Susser'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.