The model should not depend on a (concrete) DB. If it only knows an abstract DB (read "interface") which is handed to the model then you can replace the DB with a mock (http://en.wikipedia.org/wiki/Mock_object).

The model should not depend on a (concrete) DB. If it only knows an abstract DB (read "interface") which is handed to the model then you can replace the DB with a mock object.

In object-oriented programming, mock objects are simulated objects that mimic the behavior of real objects in controlled ways. A programmer typically creates a mock object to test the behavior of some other object, in much the same way that a car designer uses a crash test dummy to simulate the dynamic behavior of a human in vehicle impacts...