How to include multiple model in rails lib

In my /lib i have this class below:

module Application
  class Post < ActiveRecord::Base
     attr_accessor :id

     def artilce_content
        post.articles.content
     end

     private
        def post
           Post.find(id)
        end
  end
end

But the problem is article is undefined.

NoMethodError: undefined method ziptag_type' for #<Application::Api::V2::Ziptag:0x00000008edc120>

The question is, how can I use or include multiple models in /lib? I tried adding class User < ActiveRecord::Base; end above module

class Article < ActiveRecord::Base; end

module Application
  class Post < ActiveRecord::Base; end
        :
        :
        :
  end
end

but is doesn't worked.