Rails 4 has_many通过

Question

I have the following code in Rails 4:

  POST_MODEL_TO_INT = {text_post: 1, video_post: 2}

  class Label < ActiveRecord::Base
  end

  module PostBase
    include do
      has_many :posts_to_labels, :foreign_key => :post_id, :conditions => { post_model_id: POST_MODEL_TO_INT[self.name.underscore.to_sym] }
      has_many :labels, :through => :posts_to_labels
    end
  end

  class TextPost
    include PostBase
  end

  class VideoPost
    include PostBase
  end

  class PostsToLabel < ActiveRecord::Base
    belongs_to :post
    belongs_to :label
  end


  TextPost.first.labels => return labels collection

What must I add to the Label model to get all and each post from a label instance?

Label.first.posts # -> return collection of posts Video, Text ....
Label.first.text_posts # -> return collection of posts Text .... 

Answer 1

class Label < ActiveRecord::Base
  has_many :posts_to_labels, :conditions => { post_model_id: 2 }
  has_many :text_posts, :through => :posts_to_labels
end

class PostsToLabel < ActiveRecord::Base
  belongs_to :post
  belongs_to :label
  belongs_to :text_post, :foreign_key => :post_id
end

I find solution for single resource, but don't understood why i need add has_many to PostsToLabel ?