rails: polymorphic module

polymorphic_module.rb

##########################################################
# polymorphic as module
##########################################################

# app/models/comment.rb
#

class Comment < ActiveRecord::Base
  belongs_to :commenter, :polymorphic => true
end

module Commentable
  has_many :comments, as: :commentable
end

class Timesheet < ActiveRecord::Base
  include Commentable
end

class ExpenseReport < ActiveRecord::Base
  include Commentable
end

module Commentable
  def self.included(base)
   base.class_eval do
     has_many :comments, as: :commentable
   end
  end
end

 # app/models/concerns/commentable.rb
module Commentable
  extend ActiveSupport::Concern
  included do
    has_many :comments, as: :commentable
  end
end


# controller
class CommentsController < ApplicationController
  before_filter :find_commenter

  def create
    @comment = @commenter.comments.create(params[:comments])
    respond_to do |format|
      format.html {redirect_to :controller => @commenter.class.to_s.pluralize.downcase, :action => :show, :id => @commenter.id}
    end
  end

  private

  def find_commenter
    klass = params[:commenter_type].capitalize.constantize
    @commenter = klass.find(params[:commenter_id])
  end
end

##########################################################
##########################################################
# Combining has_many :through and polymorphic association
##########################################################

# There can be special cases where you need has_many :through with polymorphic association. Although not difficult to understand/implement, one might get confused the first time. So lets say Professor wants to know what courses/labs were assigned to his phd students/TAs. This can be made possible by doing the following.

```
# /app/models/professor.rb
class Professor < ActiveRecord::Base
  has_many :teaching_assistants
  has_many :course_tas, through: :teaching_assistants, source: :ta_duty, source_type: 'Course'
  has_many :lab_tas, through: :teaching_assistants, source: :ta_duty, source_type: 'Lab'
end
```

```
# /app/models/teaching_assistant.rb
class TeachingAssistant < ActiveRecord::Base
  belongs_to :professors
  belongs_to :ta_duty, polymorphic: true
end
```

# That’s it, we have combined has_many :through and polymorphic association. To test you can make queries like Professor.first.course_tas. Note that we have added association between Professor and TAs, so make sure you make corresponding changes.

# !!!!! When adding Bi-directional Associations in rails application, one uses inverse_of: option provided by Active Record. inverse_of: does not work with polymorphic associations.