Add support of multiple databases

[heaven-born]

I worked with the tranzactio a long time ago, and I remember an issue I could not overcome: I had more than one database and I wanted to combine two ZIOs that required two connections to two different databases and then do transactionOrDie for each of the connection separately. This is just a prototype, but I wanted something like this:

type ConnectionOne = Connection[DatabaseOne]
type ConnectionTwo = Connection[DatabaseTwo]

val list: ZIO[ConnectionOne, DbException, List[String]] = tzio {
  sql"SELECT name FROM users".query[String].to[List]
}

val list2: ZIO[ConnectionTwo, DbException, List[String]] = tzio {
  sql"SELECT name FROM users".query[String].to[List]
}

val combined:ZIO[ConnectionOne && ConnectionTwo, DbException, List[String]] = list *> list2

val foo: ZIO[DatabaseOne && DatabaseTwo, String, Long] = Database.transactionOrDie(combined)

I don't think something like this is doable in scala 2.13, but maybe in scala 3 is possible.

[guersam]

It's possible using Subtypes in ZIO Prelude in Scala 2.13, but not yet in Scala 3 due to a compiler issue or something AFAIK.

As a workaround I manually declare different subtypes of Database using inheritance. It's cumbersome but just works. Instead of extending Connection, I prefer Database for more flexibility and simplicity.

class DatabaseOne(underlying: Database) extends Database {
  override def transaction[R, E, A](...) = ???
  // other necessary overrides ...
}

[gaelrenoux]

I can definitely see how such a feature would be useful.

A simple way to make it easier would be for Tranzactio to provide directly a DatabaseWrapper that inherits Database (equivalent to your DatabaseOne). Then you could declare your own subclasses, where you would only need to extend the wrapper without having to override anything.

I could also provide a DatabaseTyped[A], with an arbitrary type-parameter that would work as a marker. It would also inherit the wrapper, and you would only need to provide a marker type. Not sure if that's better than just having you declare a subclass of DatabaseWrapper directly.

[gaelrenoux]

I created a PR for this use case. What do you think? I'm not fan of the name DatabaseT for the typed database, but couldn't find a better idea. DatabaseCustom isn't great, it's not especially custom. MyDatabase maybe?