Question

Is this possible with Rails+Mongoid:

Model.update_all("position=position-1")

Answer 1

While there’s no direct equivalent to this idiom (set field to a some function of some fields of the document), there is a way to do this exact update.

Model.collection.update({},  # find all documents
                        {'$inc' => {:position => -1}}, # decrement position
                        :multi => true) # multi-update (update all)

This is using underlying mongodb driver to do the job. I don’t know how to express this in Mongoid interface.