Callbacks now receive TrackInfo objects
In SQL, we have tables containing individual rows. Tables have strict schemas that every row must obey; there are columns with types and other possible constraints: unique, not null, value checks or references to rows of other tables. Referential integrity lies at the heart of this data approach - guarantee that if row B1 of table B references row A1 of table A, referred row (A1) must exist; orphan rows are not allowed. If we want to delete A1 row, there are two options:
，这一点在夫子中也有详细论述
but it's used for deciding which keywords you plan on using for future content
return currentUser.get();