Also I found that association_chain is not stored correctly,  it always gets the first relation from the parent document (https://github.com/aq1018/mongoid-history/blob/master/lib/mongoid/history/trackable.rb#L148). That means that if I have 2 or more relations of the same type (as in the example below), it will save always the first found:

embeds_one :address, class_name: 'Contact', as: :contactable
embeds_one :second_address, class_name: 'Contact', as: :contactable

In this example if I'm trying to track second_address document, association_chain will be :address.