class Parent
  include Mongoid::Document

  embeds_one :first_child, class_name: "Child", as: :childable
  embeds_one :second_child, class_name: "Child", as: :childable
end

class Child
  include Mongoid::Document

  embedded_in :parent, inverse_of: :childable
end

[2] pry(main)> p = Parent.create
=> #<Parent _id: 50077051312f91a556000001, _type: nil>
[3] pry(main)> p.first_child = Child.new
=> #<Child _id: 50077056312f91a556000002, _type: nil>
[4] pry(main)> p.first_child = Child.new
=> #<Child _id: 50077057312f91a556000003, _type: nil>
[5] pry(main)> p.first_child = Child.new
Mongoid::Errors::NoMetadata: 
Problem:
  Metadata not found for document of type Child.
Summary:
  Mongoid sets the metadata of a relation on the document when it is either loaded from within the relation, or added to one. The presence of the metadata is required in order to provide various functionality around relations. Most likely you are getting this error because the document is embedded and was attempted to be persisted without being associated with a parent, or the relation was not properly defined.
Resolution:
  Ensure that your relations on the Child model are all properly defined, and that the inverse relations are also properly defined. Embedded relations must have both the parent (embeds_one/embeds_many) and the inverse (embedded_in) present in order to work properly.