Sometime in the last few weeks, something changed and slave_ok no longer gets logged when doing secondary reads. I haven't had time to figure out which commit caused it, but somewhere between 739ca3905ef107c5731edd3d9b93ea3a87e4ab31 and HEAD.

MyModel.with(:read => {:mode => :secondary_preferred}).count
D, [2015-08-17T18:31:02.695249 #72908] DEBUG -- : MONGODB | localhost:27017 | appboy_development.count | STARTED | {"count"=>"my_models", "query"=>{}}