Mongoid embeds_many: push document without save in order to preserve dirty state

In Mongoid, pushing a document into an `embeds_many` relation automatically persists the document to the database. Normally, this is fine, but I run into problems when I need to track changes to the embedded document. Say you have two models: ``` class List include Mongoid::Document embeds_many :items field :title end class Item include Mongoid::Document embedded_in :list field :name end ``` This happens to the .changes attribute: ``` list = List.new(title: 'List title') list.save #list is now persisted item = Item.new(name: 'Item name') item.changes #returns Hash with {'name' => [nil, 'Item name']} list.items << item #saves item to database under the hood item.changes #returns empty Hash, because item was autosaved with list ``` I could use `item.previous_changes` to inspect the changes that were made before pushing the item into the list, but in my specific case, this would give me all kinds of troubles to keep things manageable. What I would like to achieve, is to be able to initialize an `Item` document and then add it to `list` (via `<<` or `push`) without persisting it immediately. I'm aware that Mongoid does provide an option to set up `embeds_many` relations without persisting (see http://mongoid.org/en/mongoid/docs/relations.html#embeds_many): ``` list.items.build(name: 'Another item') ``` The problem there is that Mongoid creates the Item instance for you. In my case, the documents in the `embeds_many` relation may be subclasses of Item (e.g. `SpecialItem < Item`), which wouldn't work well with `build`. But if anyone knows of a way to get around this limitation, I'd also be happy to accept it as an answer.