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node.js - Nodejs - 不确定如何在函数中等待数据库查询

问题描述

我是 nodejs 的新手，正在努力掌握 async/await 功能。我已经阅读了许多其他 SE 帖子和许多博客，但似乎无法获得正确的代码组合来实现预期的程序流程。

我的程序的总体思路是调用 startNextPick() 函数，然后调用 getLeagueFromDraftPicks()。此函数内部是一个查询，需要“等待”才能完成，但我不确定执行此操作的正确/简单方法。目前，没有等待导致函数返回“未定义”结果。在这种情况下等待查询响应的正确/简单方法是什么？

startNextPick();

async function startNextPick(pickId) {
  console.log('A-- starting next pick');

  var leagueId = await getLeagueFromDraftPick(pickId);

  console.log('B-- leagueId from getLeagueFromDraftPick: '+leagueId);

  leagueId = Number(leagueId);
  if (leagueId > 0) {
    //.... do other stuff here
  } else {
    console.log('Error with finding league based on draft pick');
  }
}

async function getLeagueFromDraftPick(pickId) {
  pickId = Number(pickId);
  var leagueId;
  console.log('1 pickId: '+pickId);
  if (pickId  <= 0) {
    return false;
  }

  // Get LeagueID from draft based on pickId
  // *** Need to "await" this query "
  connection.query('select LEAGUE_ID from DRAFT where ID=? limit 1',
                  [pickId],
                  function (error, results, fields) {
    if (error) {
      console.log(error);
      return false;
    } else {
      leagueId = results[0].LEAGUE_ID;

      console.log('2--  pickId: '+pickId);
      console.log('3--  leagueId: '+leagueId);

      return leagueId;
    }
  });
  console.log('4--  leagueId: '+leagueId);
  return leagueId;
}

使用 console.log 跟踪代码的时间和流程，很明显需要等待查询，因为 getLeagueFromDraftPick() 的返回值显示未定义，但查询结果最终显示正确。

Console.log output
****starting next pick****
A-- starting next pick
1 pickId: 19002
4--  leagueId: undefined
B-- leagueId from getLeagueFromDraftPick: undefined
Error with finding league based on draft pick
2--  pickId: 19002
3--  leagueId: 52

** 解决方案 ** 使用@cyberwombat 回答并进行一些修改，我能够让它工作。下面是工作代码：

async function startNextPick(pickId) {
  console.log('A-- starting next pick');

  var leagueId = await getLeagueFromDraftPick(pickId);

  console.log('B-- leagueId from getLeagueFromDraftPick: '+leagueId);
}


async function getLeagueFromDraftPick(pickId) {
  pickId = Number(pickId);
  console.log('1 pickId: '+pickId);
  if (pickId  <= 0) {
    return false;
  }

  // Get LeagueID from draft based on pickId
  // *** Need to "await" this query "
  const leagueId = await new Promise(function(resolve, reject)  {
    connection.query('select LEAGUE_ID from DRAFT where ID=? limit 1',
                    [pickId],
                    function (error, results, fields) {
      if (error) {
        console.log(error);
        reject(error);
      } else {
        if (results[0]) {
          console.log('2--  pickId: '+pickId);
          console.log('3--  leagueId: '+results[0].LEAGUE_ID);

          resolve(results[0].LEAGUE_ID);
        } else {
          resolve(0);        }
      }
    });
  });
  console.log('4--  leagueId: '+leagueId);
  return leagueId;
}


Console output:
****starting next pick****
A-- starting next pick
1 pickId: 19031
2--  pickId: 19031
3--  leagueId: 52
4--  leagueId: 52
B-- leagueId from getLeagueFromDraftPick: 52

标签： node.jsasynchronousasync-await

解决方案

连接位使用回调。为了让你的 async/await 可以很好地工作，你可以将它包装在一个 Promise 中（你也可以查看那个 db 库是否有一个 Promisified/async 版本）。理想情况下，您希望将其包装在 try/catch 块中以处理错误，但这是一个不同的故事。这是返回1eagueId异步的代码。

async function getLeagueFromDraftPick(pickId) {
  pickId = Number(pickId);
  var leagueId;
  console.log('1 pickId: '+pickId);
  if (pickId  <= 0) {
    return false;
  }


  const leagueId = await new Promise(function(resolve, reject)  {
    // Get LeagueID from draft based on pickId
    // *** Need to "await" this query "
    connection.query('select LEAGUE_ID from DRAFT where ID=? limit 1',
                  [pickId],
                  function (error, results, fields) {
      if (error) {
        console.log(error);
        reject(error)

      } else {
        leagueId = results[0].LEAGUE_ID;

        console.log('2--  pickId: '+pickId);
        console.log('3--  leagueId: '+leagueId);

        resolve(leagueId)
      }
    });
  })

  console.log('4--  leagueId: '+leagueId);
  return leagueId;
}

简化的可运行示例： 

// Create our async function
async function getLeagueFromDraftPick() {
  console.log('1')
  // We can use the new Promise to wrap old style callbacks - not shown here
  const leagueId = await new Promise((resolve, reject) => {
    console.log('2')
    // Fake some delay - i.e. pretend w are doing some http request
    setTimeout(() => {
      resolve('foo')
     }, 1000);
  })
  console.log('3')
  return leagueId
}

// This weird parenthesis setup is just a trick to run async functions from the console
(async () => {
  const leagueId = await getLeagueFromDraftPick()
  console.log(leagueId)
})()

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