When trying to save a project you get:
Referencing a Item document from the Project document via a relational association is not allowed since the Item is embedded.
In order to properly access a Item from Project the reference would need to go through the root document of Item. In a simple case this would require Mongoid to store an extra foreign key for the root, in more complex cases where Item is multiple levels deep a key would need to be stored for each parent up the hierarchy.
Consider not embedding Item, or do the key storage and access in a custom manner in the application code.