class Person
  include Mongoid::Document
  field :name
  embeds_many :locations, class_name: 'Location'
end

class Location
  include Mongoid::Document
  field :name
  embedded_in :person
end
class Street < Location
  field :number
end

Person.new(name: 'Peter', locations: [{_type: 'Street', number: 5}])

Mongoid::Errors::UnknownAttribute: 
message:
  Attempted to set a value for 'number' which is not allowed on the model Street.
summary:
  Without including Mongoid::Attributes::Dynamic in your model and the attribute does not already exist in the attributes hash, attempting to call Street#number= for it is not allowed. This is also triggered by passing the attribute to any method that accepts an attributes hash, and is raised instead of getting a NoMethodError.
resolution:
  You can include Mongoid::Attributes::Dynamic if you expect to be writing values for undefined fields often.