class Parent
  include Mongoid::Document
  embeds_one :embedded
end

class Embedded
  include Mongoid::Document
  embedded_in :parent
  has_many :items, as: :itemable
end

class Item
  include Mongoid::Document
  belongs_to :itemable, polymorphic: true
end

Items are only created like this

parent.embedded.items.create(...)

and that ensures that itemable_type and itemable_id are set properly.

Now, when I use Item like this

item = Item.find('42')
item.itemable
#=> nil

its itemable association always returns nil. That is completely reasonable behavior given that the itemable in question is embedded and could not be loaded through just its type and id.

However, when loading an item like this:

parent.embedded.items.find('42')

the case is slightly, but relevantly different. Here, the itemable is known beforehand and could be set from within #find.

For the time being, I have implemented the behavior in an association extension like this

class Embedded
  include Mongoid::Document
  embedded_in :parent
  has_many :items, as: :itemable do
    def find(*args)
      super.tap { |ref|
        ref.itemable = base
      }
    end
  end
end

but I'm hoping that Mongoid could handle this.