comment.rb

class Comment < ActiveRecord::Base
  belongs_to :commentable, :polymorphic => true
  has_one :user, :through => :commentable, 
end

#(rdb:1) @comment.valid?
# => true
#(rdb:1) @comment.commentable.valid?
# => true
#(rdb:1) @comment.commentable.user.valid?
# => true
#(rdb:1) @comment.user
# => NameError Exception: uninitialized constant Comment::Commentable

membership.rb

class Membership < ActiveRecord::Base
  belongs_to :user
  has_many :comments, :as => :commentable
end