Custom DatabaseValueConvertible UUID wrapper doesn’t work with scalar .select

[lunacookies]

I’m trying to create a type – MedalID in the example code below – to wrap my usages of UUID for some extra type safety. MedalID’s DatabaseValueConvertible and Codable conformances just pass everything through to the underlying UUID type. This works fine when I fetch and insert records that use the wrapper type, but errors once I try to select a single column of type MedalID (in the same way as is shown in the README).

If I swap out MedalID for a raw UUID then everything works as expected.

Self-contained example code

import Foundation
import GRDB

struct Player: Codable, FetchableRecord, PersistableRecord {
	static let medals = hasMany(Medal.self)

	var id: String
	var name: String
}

struct Medal: Codable, FetchableRecord, PersistableRecord {
	var id = MedalID()
	var playerID: String
}

struct MedalID: Codable, DatabaseValueConvertible {
	private var uuid = UUID()

	init() {}

	init(from decoder: any Decoder) throws {
		uuid = try UUID(from: decoder)
	}

	private init(_ uuid: UUID) {
		self.uuid = uuid
	}

	static func fromDatabaseValue(_ dbValue: DatabaseValue) -> MedalID? {
		UUID.fromDatabaseValue(dbValue).map(MedalID.init)
	}

	var databaseValue: DatabaseValue { uuid.databaseValue }

	func encode(to encoder: any Encoder) throws {
		try uuid.encode(to: encoder)
	}
}

func main() throws {
	let dbQueue = try DatabaseQueue()

	try dbQueue.write { db in
		try db.create(table: "player") { t in
			t.primaryKey("id", .text)
			t.column("name", .text).notNull()
		}

		try db.create(table: "medal") { t in
			t.primaryKey("id", .blob)
			t.belongsTo("player")
		}
	}

	try dbQueue.write { db in
		try Player(id: "1", name: "Arthur").insert(db)
		try Player(id: "2", name: "Barbara").insert(db)
		try Medal(playerID: "1").insert(db)
		try Medal(playerID: "1").insert(db)
		try Medal(playerID: "1").insert(db)
		try Medal(playerID: "2").insert(db)
	}

	try dbQueue.read { db in
		try debugPrint(Medal.fetchAll(db)) // This decodes as you'd expect.

		struct PlayerInfo: Decodable, FetchableRecord {
			var player: Player
			var medalIDs: [MedalID]

			static func all() -> QueryInterfaceRequest<PlayerInfo> {
				Player
					.including(all: Player.medals.select(Column("id")).forKey(CodingKeys.medalIDs))
					.asRequest(of: PlayerInfo.self)
			}
		}

		// could not decode String from database value Data(16 bytes) -
		// column: "id", column index: 0, row: [id:Data(16 bytes) grdb_playerId:"1"]
		try debugPrint(PlayerInfo.all().fetchAll(db))
	}
}

try! main()

Thanks so much for GRDB!

[groue]

Hello @lunacookies,

Thanks for asking. The sample code looks valid and innocuous to me. I'll have a look.

[groue]

OK. I understand where the problem comes from.

While decoding PlayerInfo from a database row, it is the Decodable conformance of MedalID that is used to decode the values loaded from the database, not its DatabaseValueConvertible conformance:

// Decodable conformance runs
init(from decoder: any Decoder) throws {
    uuid = try UUID(from: decoder)
}

// DatabaseValueConvertible conformance does not run
static func fromDatabaseValue(_ dbValue: DatabaseValue) -> MedalID? {
    UUID.fromDatabaseValue(dbValue).map(MedalID.init)
}

Unfortunately, the Decodable facet of UUID has no support for blobs. It requires a String, fails to decode one from the database blob (as expected - a binary UUID is not the representation of a string), hence the error your can see.

---

🐛 This is a bug in GRDB. An oversight. The intent of the library is to always favor database protocol conformances over Codable conformances when both are available. This code has to be updated so that it favors FetchableRecord and DatabaseValueConvertible over Decodable: https://github.com/groue/GRDB.swift/blob/v7.6.1/GRDB/Record/FetchableRecord%2BDecodable.swift#L317-L321

When the GRDB bug is fixed, and it is the DatabaseValueConvertible conformance of MedalID that is preferred, it will correctly decode the database blob.

---

What are your options, until the bug is fixed?

As a temporary workaround, update MedalID.init(from decoder: any Decoder) so that it tries to decode Data, and a UUID from the decoded data (possible implementation), if decoding UUID fails.

[lunacookies]

Ahh, that makes perfect sense! Thank you so much for taking the time to figure this out.

[groue]

You're welcome! I'll tell you when the bug is fixed!