Can join tables be polymorphic?

rubyonrails-talk
1

For example: Currently I have: functions < functions_competencies < competencies exams < exam_competencies < competencies

I'd like to dry up "functions_competencies" and "exam_competencies" by using a polymorphic association, by let's say 'attached_competencies':

functions < attached_competencies < competencies exams < attached_competencies < competencies evaluations < attached_competencies < competencies ...

Note that these are join tables.

Is this possible?

2

This is only somewhat possible - you can do a has_many :through from the specific classes to the general, but the other direction will get you an exception. In your case:

in function.rb:

has_many :attached_competencies, :as => :competency_thing, :dependent => :destroy has_many :competencies, :through => :attached_competencies # this works

in attached_competency.rb:

belongs_to :competency_thing, :polymorphic => true belongs_to :competency

in competency.rb:

has_many :attached_competencies, :dependent => :destroy has_many :???, :through => :attached_competencies # THIS WILL NOT WORK

You *can* do a through for a particular type (in competency.rb again):

has_many :functions, :through => :attached_competencies, :source => :competency_thing, :source_type => 'Function'

Which may or may not be sufficient for your purposes.

--Matt Jones

3

Andy Tolle wrote in post #991031:

For example: Currently I have: functions < functions_competencies < competencies exams < exam_competencies < competencies

I'd like to dry up "functions_competencies" and "exam_competencies" by using a polymorphic association, by let's say 'attached_competencies':

functions < attached_competencies < competencies exams < attached_competencies < competencies evaluations < attached_competencies < competencies ...

Note that these are join tables.

Is this possible?

IIRC, this is based on the has_many_polymorphs plugin

class appl   has_many :appllinks   has_many :projects,            :through => :appllinks,            :source => :project,            :conditions =>? "appllinks.appllinkable_type = 'Project'"   has_many :scenarios,            :through => :appllinks,            :source => :scenario,            :conditions =>? "appllinks.appllinkable_type = 'Scenario'"   has_many :unittests,            :through => :appllinks,            :source => :unittest,            :conditions =>? "appllinks.appllinkable_type = 'Unittest'"

class appllink   belongs_to :appl   belongs_to :appllinkable, :polymorphic => true   belongs_to :project,              :class_name => 'Project',              :foreign_key => 'appllinkable_id'   belongs_to :scenario,              :class_name => 'Scenario',              :foreign_key => 'appllinkable_id'   belongs_to :unittest,              :class_name => 'Unittest',              :foreign_key => 'appllinkable_id'

class project   has_many :appllinks, :as => :applinkable, :dependent => :destroy   has_many :appls, :through => :appllinks

classes scenario and unittest are just like project

YMMV