Request是Container，它有eventLoop，它是DatabaseConnectable。

Application，它是Container，它有eventLoop，但不是DatabaseConnectable。

如何使用Application进行数据库查询？

在任何Container上，您都可以建立到数据库的池连接，而您可能猜想的连接是DatabaseConnectable。

boot.swift中的示例查询

import Vapor
import FluentSQL
import FluentPostgreSQL

public func boot(_ app: Application) throws {
    let _ = app.requestPooledConnection(to: .psql).flatMap { conn in
        return User.query(conn).all().map { users in
            print("just found \(users.count) users")
        }.always {
            try? app.releasePooledConnection(conn,to: .psql)
        }
    }
}

上面的代码将通过.psql容器中的Application标识符请求到PostgreSQL的池化连接，然后对该连接执行查询，然后在always块中将其释放回池。

在哪里获得Container的后台任务？

如果使用https://github.com/vapor/jobs或https://github.com/MihaelIsaev/VaporCron，则在任务声明中将有一个Container对象。

VaporCron的示例

// in boot.swift
import Vapor
import VaporCron

/// Called after your application has initialized.
public func boot(_ app: Application) throws {
    scheduleTasks(on: app)
}

/// Scheduling Cron tasks
private func scheduleTasks(on app: Application) {
    do {
        _ = try VaporCron.schedule(Every1MinCheck.self,on: app)
    } catch {
        print("cron schedule error: \(error)")
    }
}

// in Every1MinCheck.swift
import Vapor
import VaporCron
import FluentSQL
import PostgreSQL

struct Every1MinCheck: VaporCronSchedulable {
    static var expression: String { return "*/1 * * * *" } // every 1 minute

    static func task(on container: VaporCronContainer) -> Future<Void> {
        return container.requestPooledConnection(to: .psql).flatMap { conn in
            return User.query(conn).all().map { users in
                print("just found \(users.count) users")
            }.always {
                try? container.releasePooledConnection(conn,to: .psql)
            }
        }
    }
}

基于这个问题和答案（Is is possible to use Vapor 3 Postgres Fluent in a standalone script?），我意识到CRUD就像这样：

import Vapor
import FluentPostgreSQL

final class Device: PostgreSQLModel {
    var id: Int?
    var isWorking: Bool
    var serial: Int

    init(isWorking: Bool,serial: Int) {
        self.isWorking = isWorking
        self.serial = serial
    }
}
extension Device: Content {}
extension Device: Migration {}
extension Device: Parameter {}

final class WorkWithPostgres {

    let databaseConfig = PostgreSQLDatabaseConfig(hostname: "localhost",port: 5432,username: "username",database: "testestest",password: nil)

    let database: PostgreSQLDatabase

    static let shared = WorkWithPostgres()

    private init() {
        database = PostgreSQLDatabase(config: databaseConfig)
    }

    func readAll<T: PostgreSQLModel>(postgreSQLModel: T.Type,completion: (([T]) -> Void)?) {
        let worker = MultiThreadedEventLoopGroup(numberOfThreads: 1)
        let conn = database.newConnection(on: worker)

        let _ = conn.map { connection in
            postgreSQLModel.query(on: connection).all().map { databaseData in
                worker.shutdownGracefully { _ in
                }

                completion?(databaseData)
            }
        }
    }

    func create<T: PostgreSQLModel>(postgreSQLModel: T) {
        let worker = MultiThreadedEventLoopGroup(numberOfThreads: 1)
        let conn = database.newConnection(on: worker)

        let _ = conn.map { connection in
            let _ = postgreSQLModel.save(on: connection).whenComplete {
                worker.shutdownGracefully { _ in
                }
            }
        }
    }

}

final class DeviceController {

    func readAll(completion: (([Device]) -> Void)?) {
        WorkWithPostgres.shared.readAll(postgreSQLModel: Device.self) { devices in
            completion?(devices)
        }
    }

    func create(isWorking: Bool,serial: Int) {
        let device = Device(isWorking: isWorking,serial: serial)

        WorkWithPostgres.shared.create(postgreSQLModel: device)
    }

}

它正在工作，但是我不确定这是个好方法。有人知道吗？