To clarify, when I wrote "the ways units interact", I was referring to units that represent external dependencies in some kind (often a database, as you said).

To clarify, when I wrote "the ways units interact", I was referring to units that represent external dependencies in some kind (often a database, as you said). Many refactorings change those interactions in some way. I agree that there is no reason to mock data containers or other classes with fully self-contained behaviour.