         do {
             try dbQueue.inTransaction { db in
                 do {
-                    try Artist(name: "Gerhard Richter").save(db)
+                    try Artist(id: nil, name: "Gerhard Richter").save(db)
                 } catch {
                     XCTFail("Unexpected Error")
                 }

        let observer = Observer()
        dbQueue.add(transactionObserver: observer)
        
        let artist = Artist(name: "Gerhard Richter")
        let artwork1 = Artwork(title: "Cloud")
        let artwork2 = Artwork(title: "Ema (Nude on a Staircase)")
        
        try dbQueue.inTransaction { db in
            try artist.save(db)

        
        try dbQueue.inTransaction { db in
            do {
                try Artist(id: nil, name: "Vincent Fournier").save(db)
            } catch {
                XCTFail("Unexpected Error")
            }
