This code now errors out on the 7.0.x Mongoid version. I ran this code on 6.4.2, where it works, but it errors out on 7.0.0, 7.0.1, and 7.0.2.

class TestModel
  include Mongoid::Document  embeds_many :other_models, :class_name => "OtherModel", :store_as => "x"
end

class OtherModel
  include Mongoid::Document
  embedded_in :test_model
end

m = TestModel.create
m.other_models.create
m.dup() 

It errors with
NoMethodError: undefined method `each_with_index' for nil:NilClass
from /Users/jon/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/mongoid-7.0.0/lib/mongoid/copyable.rb:76:in `block in process_localized_attributes'

because the code at https://github.com/mongodb/mongoid/blob/master/lib/mongoid/copyable.rb#L77 errors because it does not use the "store_as" value to find the attribute.