1 - The autobuild: true
option normally should have done the trick. I think the problem is that you forgot to add the other side of the relation to the Thingamajig model:
class Thingamajig
include Mongoid::Document
embedded_in :user
...
end
2 - To specify required fields, use validations:
class User
include Mongoid::Document
field :name, type: String
validates_presence_of :name
...
end
Mongoid uses ActiveModel validations.