Using polymorph many-to-many self referencing relation with attributes on the relation in rails

Question

Hi, I'd like to create a self referencing relation in rails. I have a Person model, and the person should have masters and pupils with same Person object.

So far I tried:

class Person <ActiveRecord::Base
   has_many :relationships, :dependent => :destroy
   has_many :masters, :through => :relationships, :conditions => "status='master'"
   has_many :pupils, :through => :relationships, :conditions => "status='pupil'"
   has_many :inverse_relationships, :class_name => "Relationship",
      :foreign_key => "related_id"
   has_many :inverse_masters, :through => :inverse_relationships,  
      :source => :person, :conditions => "status='master'"
   has_many :inverse_pupils, :through => :inverse_relationships,  
      :source => :person, :conditions => "status='pupil'"
end

class Relationship < ActiveRecord::Base
  belongs_to :person
  belongs_to :master, :class_name => "Person", :foreign_key => 'related_id'
  belongs_to :pupil, :class_name => "Person", :foreign_key => 'related_id'
end

It seems to work when I am trying to select:

@a = Person.find(:first)
@a.masters

but when I try to do a push into masters, it saves the relationship without the status set to master. It saves null instead. Is there an easy way to save status=master when I push into masters and status=pupil when I push into pupils?

Thanks

Answer 1

To make it short the solution is: association callbacks (more here under the Association Callback section: http://railsapi.com/doc/rails-v3.0.0/classes/ActiveRecord/Associations/ClassMethods.html)

To be a little more detailed I have adapted your example a little bit, but basically the structure is the same, here is the code:

class Person < ActiveRecord::Base
  has_many :relationships
  has_many :pupils, :through => :relationships, :source => :other_person, :conditions => 'relationships.type = "MasterPupil"', :after_add => Proc.new{|p,o| Relationship.update_all("type = 'MasterPupil'", ['person_id = ? AND other_person_id = ?', p.id, o.id])}
  has_many :masters, :through => :relationships, :source => :other_person, :conditions => 'relationships.type = "PupilMaster"', :after_add => Proc.new{|p,o| Relationship.update_all("type = 'PupilMaster'", ['person_id = ? AND other_person_id = ?', p.id, o.id])}
end

class Relationship < ActiveRecord::Base
  belongs_to :person
  belongs_to :other_person, :class_name => 'Person'
  before_validation :set_type

  def set_type
    self.type = 'OpenRelationship'
  end
end

class MasterPupil < Relationship
end

class PupilMaster < Relationship
end

The RelationShip model contains a type column which is the equivalent of your status column, but type is nicer if I later want to do an STI and declare MasterPupil/PupilMaster relationship models.

RelationShip also has a set_type before_validation that will set the type to OpenRelationship which should be temporary before the after_add callback defined in the Person model in each association will set things clear (and set either a MasterPupil or PupilMaster type)

and now:

Loading development environment (Rails 3.0.0)
irb(main):001:0> p = Person.create
=> #<Person id: 1, created_at: "2010-09-10 23:35:37", updated_at: "2010-09-10 23:35:37">
irb(main):002:0> p.pupils
=> []
irb(main):003:0> p.masters
=> []
irb(main):004:0> p.pupils << Person.create
=> [#<Person id: 2, created_at: "2010-09-10 23:35:56", updated_at: "2010-09-10 23:35:56">]
irb(main):005:0> Relationship.all
=> [#<MasterPupil id: 1, person_id: 1, other_person_id: 2, type: "MasterPupil">]
irb(main):006:0> p.masters << Person.create
=> [#<Person id: 3, created_at: "2010-09-10 23:36:29", updated_at: "2010-09-10 23:36:29">]
irb(main):007:0> Relationship.all
=> [#<MasterPupil id: 1, person_id: 1, other_person_id: 2, type: "MasterPupil">, #<PupilMaster id: 2, person_id: 1, other_person_id: 3, type: "PupilMaster">]
irb(main):008:0> p.reload
=> #<Person id: 1, created_at: "2010-09-10 23:35:37", updated_at: "2010-09-10 23:35:37">
irb(main):009:0> p.pupils
=> [#<Person id: 2, created_at: "2010-09-10 23:35:56", updated_at: "2010-09-10 23:35:56">]
irb(main):010:0> p.masters
=> [#<Person id: 3, created_at: "2010-09-10 23:36:29", updated_at: "2010-09-10 23:36:29">]