Not sure if it’s idiomatic, but I find it helpful reference a function from my event / subscription. It also makes it easier to test.

(defn event-name
  "event-name's doc"
  [{:keys [db]} _]
  {:db db})

(re-frame/reg-event-fx :event-namespace/event-name event-name)

https://github.com/Day8/re-frame/blob/master/examples/todomvc/src/todomvc/subs.cljs#L90 in a layer 3 subscription, should i add a signal function that returns the app db if i still need the db in my computation function, or should i just use re-frame.db/app-db directly?

i guess i need the app-db as a signal, so when it changes, the sub recomputes?

It looks to me like the computation function is passed the result of the signal function

So you grab any data you need out of db in the signal, and it comes to computation as a parameter

yeah, so i guess it doesn't make sense (from a performance perspective) to pass the db through 🙂 ok, talked myself out of it

or else, just use the regular old layer 2, and unpack what you need from the db

Yea that seems about right. That page seems to indicate that the layer-3 is only really useful to prevent re-computing an expensive derivation function every time db is changed