hello world does not wirte to disk ?

[expll]

"hello world" code can run, but when I terminal my app and just run the code below, str is nil

let db = YapDatabase()
db.registerCodableSerialization(String.self, forCollection: "test")

let connection = db.newConnection()

//connection.readWrite {(transaction) in
// transaction.setObject("hello", forKey: "world", inCollection: "test")
//}

connection.read {(transaction) in
let str = transaction.object(forKey: "world", inCollection: "test") as? String
}