The callback here is okay. I don't feel like this is the appropriate place to change the host, however. I think it would make more sense to do that sort of switching in e.g. a [beforeRequest](https://github.com/CakePHP-Copula/Copula/blob/master/Model/Datasource/ApisSource.php#L367) callback. Possibly the default behavior for that should also include a check to see if the model has a method named beforeRequest() -- which was likely the thinking behind this [otherwise pointless bit of code](https://github.com/CakePHP-Copula/Copula/blob/master/Model/Datasource/ApisSource.php#L185). The idea behind extras was for it to be passed directly to Router::querystring(), which can take additional arguments. I don't entirely trust my memory, but I suspect that using `$extras` in this manner will append the api domain to the query string as well as change the host.