How to save changes on update?

How do I save only changes in the database?

I don't want to save the whole document. I want to save only changes is there a way to do that?

for example: 

a = {"foo":"1","bar":"2","foobar":"3"}

now if I update a, so that a = {"foo":"2","bar":"2","foobar":"3"}

how to only store a= {"foo":"2"}