class Like
  include Mongoid::Document
  belongs_to :likable, polymorphic: true
  belongs_to :user
end

class Submission
  include Mongoid::Document

  belongs_to :creator, class_name: 'User', inverse_of: :submissions
  has_many :likes, :as => :likable, :dependent => :destroy
end

class User
  include Mongoid::Document
  has_many :submissions, :dependent => :destroy

  has_many :liked_submissions, :as => :likable, :dependent => :destroy, class_name: 'Like'
  has_many :liked_comments, :as => :likable, :dependent => :destroy, class_name: 'Like'
end