Question

Does anyone know how to do a polymorphic association in Mongoid that is of the relational favor but not the embedding one.

For instance this is my Assignment model:

class Assignment
  include Mongoid::Document
  include Mongoid::Timestamps

  field :user
  field :due_at, :type => Time

  referenced_in :assignable, :inverse_of => :assignment
end

that can have a polymorphic relationship with multiple models:

class Project
  include Mongoid::Document
  include Mongoid::Timestamps

  field :name, :type => String

  references_many :assignments
end

This throws an error saying unknown constant Assignable. When I change the reference to embed, this all works as documented in Mongoid’s documentation, but I need it to be reference.

Thanks!

Answer 1

From Mongoid Google Group it looks like this is not supported. Here’s the newest relevant post I found.

Anyway, this is not to hard to implement manually. Here’s my polymorphic link called Subject.

Implementing inverse part of relation might be somewhat more complicated, especially because you will need same code across multiple classes.

class Notification
  include Mongoid::Document
  include Mongoid::Timestamps

  field :type, :type => String
  field :subject_type, :type => String
  field :subject_id, :type => BSON::ObjectId

  referenced_in :sender, :class_name => "User", :inverse_of => :sent_notifications
  referenced_in :recipient, :class_name => "User", :inverse_of => :received_notifications

  def subject
    @subject ||= if subject_type && subject_id
      subject_type.constantize.find(subject_id)
    end
  end

  def subject=(subject)
    self.subject_type = subject.class.name
    self.subject_id   = subject.id
  end
end