How to safely modify in memory mongoid model?

I'm using mongoid gem on rail project and I'm quite puzzled by trying to modify the model in memory but never saving it so I do not modify the db. I'm trying to modify an attribute from a model loaded in memory but it does not work as shown bellow:

mymodel = MyModel.where('some criteria')
mymodel.first.some_attribute = 0
mymodel.first.some_attribute == 0 -> is false

So I guess mongo reloads from the db each time we do first or even looping on each entry and setting some attribute has no effect, if I loop again all attributes I set are still set to the original value. Is there a way to commit the transaction and force mymodel to stay loaded in memory? It's hard for me to use proper terminology so I hope you get what I'm saying.