Don't mock stuff that's not needed

cut = NEW /clean/class_under_test( db_reader = db_reader
                                   config    = VALUE #( )
                                   writer    = VALUE #( ) ).

Define your givens as precisely as possible: don’t set data that your test doesn’t need, and don’t mock objects that are never called. These things distract the reader from what’s really going on.

" anti-pattern
cut = NEW /dirty/class_under_test( db_reader = db_reader
                                   config    = config
                                   writer    = writer ).

There are also cases where it’s not necessary to mock something at all - this is usually the case with data structures and data containers. For example, your unit tests may well work with the production version of a transient_log because it only stores data without any side effects.