A request to clarify a behavior related to upsert$

[lilsweetcaligula]

I have code like this:

si.make('players')
  .data$({ id: 'MyPrecious', username: 'elvis', points: 9999 }) // => We are assuming the matching entity exists.
  .save$({ upsert$: ['username'] }, done)

Let's assume, a matching entity exists (i.e. there already exists a player with the username 'elvis'). What behavior should I expect?
Now let's assume, no matching entity exists. What behavior should I expect?