Page 359: “when Rails performs a save it only saves the attributes which have been modified.” This isn’t quite accurate and doesn’t explain why you might want to turn partial updates off or use _will_change. Rails saves attributes which have been changed by direct assignment, but not if they have been modified some other way eg:
user.name = 'steve' user.save user.name = 'S'
You have an example of this on page 360 but it’s not actually explained in the text that Rails doesn’t ‘notice’ a change applied by ‘«’ (or anything other than attrib=). There are a lot of post-2.x upgrade questions in forums to which this is the answer and one extra sentence to explain how Rails notices a modified attribute might help.