javascript - 解析云函数-按顺序执行的代码

Question

如果满足某些条件，并且在 for 循环结束时，我有以下函数来更新行，响应包括更新了多少行。尽管更新了超过零行，但响应显示为零。查看日志，似乎 reponse.success() 在完成 for 循环之前触发。

为什么会这样？

Parse.Cloud.define("reset", function(request, response) {


    var isSaveNeeded = false
  var Query = new Parse.Query("price");
  Query.equalTo('isActive', true);
  Query.find({useMasterKey:true})
      .then((results) => {
        console.log("Found " + results.length + " price rows")
        var currentDate = moment()
        var noOfRowsUpdated = 0
        for (let i = 0; i < results.length; ++i) {
            var valid_till_date = results[i].get('valid_till_date');

            if (valid_till_date == null) {
                // if user had not selected valid_till_date then set to expire after system max no of days
                var updatedAt = results[i].get('updatedAt');
                if (currentDate.diff(updatedAt,'days') > 10) {
                  console.log("Permanent change row to be set inactive. Updated at - " + currentDate.diff(updatedAt)+ updatedAt)
                    results[i].set('isActive',false)
                    isSaveNeeded = true
                }
            } else if (currentDate.diff(valid_till_date) > 0) {
                // check whether valid_till_date has passed
                console.log("Found row with elapsed valid date "  + results[i].id)
                results[i].set("isActive",false)
                isSaveNeeded = true
            }
            if (isSaveNeeded == true) {
              console.log("Record needs to be saved for " + results[i].id)
              results[i].save(null, {useMasterKey:true})
                .then(function (user) {
                    ++noOfRowsUpdated
                    console.log("reset : Object ID: " + results[i].id + " saved - " + noOfRowsUpdated)
                  })
                .catch(function (error) {
                    console.log("reset : Error saving Object ID: " + results[i].id + error);
                    response.error(error);
                })
            } else {

              console.log("Record not to be saved for " + results[i].id)
            }
            isSaveNeeded = false
        } // end of for loop
        //BELOW IS EXECUTED BEFORE FOR LOOP COMPLETES
        console.log("Updated " + noOfRowsUpdated +" price rows");
        response.success("Updated " + noOfRowsUpdated +" price rows")
      }) // end of .then((results) 
      .catch(function(error) {
        response.error("Failed to fetch from price" + error );
      });
});

Answer 1

Parse.com 的 save 运行异步，因此循环在保存发生之前完成。解决方案是稍微重新组织一下代码，并等待保存发生后再执行响应函数。

诀窍是收集数组中每个保存返回的 promise ，并使用 Promise.when() (Promise.all() 的同义词)等待这些 promise 的履行

为了更清楚地说明，去掉“是否需要保存”逻辑，因此这个云函数只能用于处理数据库......

Parse.Cloud.define("reset", function(request, response) {
    var Query = new Parse.Query("price");
    Query.equalTo('isActive', true);
    Query.find({useMasterKey:true}).then((results) => {
        console.log("Found " + results.length + " price rows");
        // assuming ES6 or something like underscore
        let pricesToSave = results.filter(price => priceNeedsSave(price));

        // here's the important part, collect the promise from each save
        // proceed only after the promises have completed
        let promises =  pricesToSave.map(price => price.save(null, {useMasterKey:true}));
        return Parse.Promise.when(promises).then(() => pricesToSave.length);
    }).then(count => {
        response.success("Updated " + count +" price rows");
    }).catch(error => {
        response.error("Failed to fetch from price" + error );
    });
}

为了完整起见，下面是分解出的 needSave 逻辑。 (OP应该检查一下，我刚刚复制了循环体)...

function priceNeedsSave(price) {
    var isSaveNeeded = false;
    var currentDate = moment()

    var valid_till_date = price.get('valid_till_date');
    if (valid_till_date == null) {
        // if user had not selected valid_till_date then set to expire after system max no of days
        var updatedAt = price.get('updatedAt');
        if (currentDate.diff(updatedAt,'days') > 10) {
          console.log("Permanent change row to be set inactive. Updated at - " + currentDate.diff(updatedAt)+ updatedAt)
            price.set('isActive',false)
            isSaveNeeded = true
        }
    } else if (currentDate.diff(valid_till_date) > 0) {
        // check whether valid_till_date has passed
        console.log("Found row with elapsed valid date "  + price.id)
        price.set("isActive",false)
        isSaveNeeded = true
    }
    return isSaveNeeded;
}