GraphQL (and falcor) was created for exactly this reason, combining data from disparate sources, making it easy to get what you want from the frontend.
This implementation requires one to write schema, that is in many ways (maybe fully) identical to your database schema e.g. Slick schema thus you have to write it twice. It would be a better to infer the GraphQL schema from Slick/database schema.
That being said, there's a lot of low hanging fruit for building a GraphQL schema "generator" libraries for various backing databases - while some things are likely to change, it's quite nice to just apply those edits rather than building up a near-parallel schema again.