Identity Field

Saves a database ID field in an object to maintain identity between an in-memory object and a database row.

Relational databases tell one row from another by using key - in particular, the primary key. However, in-memory objects don't need such a key, as the object system ensures the correct identity under the covers (or in C++'s case with raw memory locations). Reading data from a database is all very well, but in order to write data back you need to tie the database to the in-memory object system.

In essence, Identity Field is mind-numbingly simple. All you do is store the primary key of the relational database table in the object's fields.

for more details go to Chapter 12 of the online ebook at oreilly.com