I used to do this in my own application years ago when we supported multiple databases. Short answer is you don’t, long answer is you will have to write your own custom queries using Knex or some other query builder because at this time, from what I’m aware of, we do not intend to implement multiple database support in a single application for the short to medium term future.
If anything we might consider it if we decide to add multi-tenancy. @sunnyson had similar concerns and he might be able to share some of his workarounds as he is doing exactly this right now.