Discussion: Use Codec Composition for Joins

[sinanspd]

Currently schemas that require a Join query are defined similar to the following:

 val decoder: Decoder[Item] =
    (itemId ~ itemName ~ itemDesc ~ money ~ brandId ~ brandName ~ categoryId ~ categoryName).map {
      case i ~ n ~ d ~ p ~ bi ~ bn ~ ci ~ cn =>
        Item(i, n, d, p, Brand(bi, bn), Category(ci, cn))
    }

I would like to argue that this doesn't scale well and isn't very pretty (at least at scale). Given that we already have Codecs for Brand and Category so I think this Decoder should be defined in terms of composition (which as functional idea is also worth iterating once again here).

This would require the SQL objects to be at least package private

Opening the issue for discussion but happy to make a PR if people are open to it