javascript - 无法访问 for 循环中使用的链式 promise 内的外部作用域变量

Question

我需要在 async/await 里面做 for..loop ，但那样我就不能做并行操作。
所以我将这些 promise 插入一个变量 promises 稍后我将在它上面做 promise.all()。这是我的代码。

const promises = [];
  for (let i = 0; i < todayAssignedJobs.length; i += 1) {
    promises.push(Leaderboard.findOrCreate({...}).then((leaderboard, created) => {
      if (!created) {
        const rating = (todayAssignedJobs[i].rating + leaderboard.rating * leaderboard.jobs_completed) / (leaderboard.jobs_completed + 1);
        const commission = todayAssignedJobs[i].commission + leaderboard.commission;
        const jobsCompleted = leaderboard.jobs_completed + 1;
        Leaderboard.update({
          rating,
          commission,
          jobs_completed: jobsCompleted,
          updated_by: 'system',
        }, {
          where: {
            id: leaderboard.id,
          },
        });
      }
      AssignedJob.update({
        is_leaderboard_generated: true,
      }, {
        where: {
          id: todayAssignedJobs[i].id,
        },
      });
    }));
  }
await Promise.all(promises)

不知何故，我在执行 Assigned.update({..}, { where: { id: todayAssignedJobs[i].id }}) 时无法获得 id

获取错误:

Unhandled rejection Error: WHERE parameter "id" has invalid "undefined" value

有人可以解释发生了什么吗？另外，请建议我可以在下面做吗？

promises.push(async () => { // I will use await here })

Answer 1

只是对现有答案的一些进一步改进。
Leaderboard.update() 和 AssignedJob.update() 是需要等待的异步函数，因此需要将回调函数转换为 async 函数。这确保在所有数据库操作完成之前不会解析 promise ，而不仅仅是 findOrCreate() :

const promises = todayAssignedJobs.map(async todayAssigned => {
  const [leaderboard, created] = await Leaderboard.findOrCreate({...});

  if (!created) {
    const rating = (todayAssigned.rating + leaderboard.rating * leaderboard.jobs_completed) / (leaderboard.jobs_completed + 1);
    const commission = todayAssigned.commission + leaderboard.commission;
    const jobsCompleted = leaderboard.jobs_completed + 1;

    await Leaderboard.update({
      rating,
      commission,
      jobs_completed: jobsCompleted,
      updated_by: 'system',
    }, {
      where: {
        id: leaderboard.id,
      },
    });
  }

  await AssignedJob.update({
    is_leaderboard_generated: true,
  }, {
    where: {
      id: todayAssigned.id,
    },
  });
});

await Promise.all(promises);

这种方法的一个更基本的问题是 Leaderboard.findOrCreate() 和 Leaderboard.update() 不是单个事务的一部分。这是有问题的，因为 update() 取决于 leaderboard 中条目的当前值，由于条目的 non-atomic 修改，这会在您的数据库中创建竞争条件:

const rating = (todayAssigned.rating + leaderboard.rating * leaderboard.jobs_completed) / (leaderboard.jobs_completed + 1);
const commission = todayAssigned.commission + leaderboard.commission;
const jobsCompleted = leaderboard.jobs_completed + 1;

每个方法都需要标记为单个事务的一部分。使用 sequelize.js，您可以使用 managed transaction 来实现:

const promises = todayAssignedJobs.map(
  todayAssigned => sequelize.transaction(async transaction => {
    const [leaderboard, created] = await Leaderboard.findOrCreate({
      transaction,
      ...
    });

    if (!created) {
      const rating = (todayAssigned.rating + leaderboard.rating * leaderboard.jobs_completed) / (leaderboard.jobs_completed + 1);
      const commission = todayAssigned.commission + leaderboard.commission;
      const jobsCompleted = leaderboard.jobs_completed + 1;

      await Leaderboard.update({
        rating,
        commission,
        jobs_completed: jobsCompleted,
        updated_by: 'system',
      }, {
        transaction,
        where: {
          id: leaderboard.id,
        },
      });
    }

    await AssignedJob.update({
      is_leaderboard_generated: true,
    }, {
      transaction,
      where: {
        id: todayAssigned.id,
      },
    })
  })
);

await Promise.all(promises);