In ActiveRecord, the #create_with method is extremely useful in conjunction with #find_or_create_by. Example:
Currently, as I understand it, the only way to implement this in Mongoid is to pass a block (which you can also do in AR, of course).
It would be great if #create_with were supported. Two reasons:
1. It's more readable. Nothing in the block syntax tells you that the block is only executed if the object is created and not if it is found already existing; in fact, there's every reason for those unfamiliar with the details of #find_or_create_by to assume that the block is always executed. The #create_with syntax makes it clearer what is executed when.
2. This would further ease transition from ActiveRecord.