[RonnieOwnsLexus]

i dont get it. I if am taking a dependency on database or another class and i mock it using its interface, what is the harm in it? Essentially i have tested that given my dependencies working correctly my class would also work as expected.

[rkeene2]

Almost -- you're testing that given your mocking implementation perfectly mirrors what the dependency would do given the inputs tested with that your functions produce the correct outputs (and hopefully you also verified the side-effects).

The article is stating that almost nobody goes through the trouble of implementing a mock database perfectly, they just do something like make a single call return some hard-coded data. While this works a bit, it means that if the database ever changes its interface you have to remember to notice and implement that change as well.