Transactions for Vapor 4 alpha 3

Database+transaction.swift

import FluentPostgresDriver
import FluentSQLiteDriver

// This extension adds transaction support to the Database protocol.
// It has to be specialized for each Database type used by your application,
// since the Database protocol does not support the raw() method.

extension Database {
    func transaction(callback: @escaping (Database) -> EventLoopFuture<Void>) -> EventLoopFuture<Void> {
        return self.withConnection { (db) -> EventLoopFuture<Void> in
            switch db {
            case let db as SQLiteConnection:
                return db.raw("BEGIN TRANSACTION").run()
                    .flatMap { callback(db) }
                    .flatMap { db.raw("COMMIT").run() }
                    .flatMapError { (err) in
                        db.raw("ROLLBACK").run()
                            .flatMap { () -> EventLoopFuture<Void> in
                                return db.eventLoop.makeFailedFuture(err)
                        }
                }
            case let db as PostgresConnection:
                return db.raw("BEGIN TRANSACTION").run()
                    .flatMap { callback(db) }
                    .flatMap { db.raw("COMMIT").run() }
                    .flatMapError { (err) in
                        db.raw("ROLLBACK").run()
                            .flatMap { () -> EventLoopFuture<Void> in
                                return db.eventLoop.makeFailedFuture(err)
                        }
                }
            default:
                fatalError("Unsupported Database type for transaction: \(type(of: db))")
                // If you would prefer to forgo the safety of the transaction
                // and simply execute the callback, you can replace the fatalError with:
                //    return callback(db)
            }
        }
    }
}