node.js 中的 foreach 异步函数

Question

我想遍历每个学生，然后执行两个查询，这两个查询的执行应该是同步的，首先是第一个，然后因为第二个查询取决于第一个。

我已经写了一些代码，但它似乎根本不起作用：

Student.find({ status: 'student' })
    .populate('student')
    .exec(function (err, students) {
        if (err) {
            return res.status(400).send({
                message: errorHandler.getErrorMessage(err)
            });
        }

        _.forEach(students, function (student) {
            async.waterfall(
                [
                    function (callback) {
                        console.log('first ' + student.firstName);
                        Student.find({ "_id": student.id }, callback);
                    },
                    function (student, callback) {
                        console.log('second '+ student[0].firstName);
                        WorksnapsTimeEntry.find({
                            "student": {
                                "$in": student.map(function (el) {
                                    return el._id
                                })
                            }
                        }, callback);
                    }
                ],
                function (err, results) {
                    if (err) {
                        // do something
                    } else {
                        // results are the matching entries
                        console.log('third');
                        var totalMinutes = 0;
                        var totalAvgLevelActivity = 0;
                        var counter = 0;
                        _.forEach(results, function (item) {
                            _.forEach(item.timeEntries, function (item) {
                                if (item.duration_in_minutes) {
                                    totalMinutes = totalMinutes + parseFloat(item.duration_in_minutes[0]);
                                }

                                if (item.activity_level) {
                                    totalAvgLevelActivity = totalAvgLevelActivity + parseFloat(item.activity_level[0]);
                                    counter++;
                                }
                            });
                        });

                        var obj = {};
                        obj.studentId = 'test';
                        obj.firstName = 'test';
                        obj.lastName = 'test';
                        obj.municipality = 'test';
                        obj.totalMinutes = totalMinutes;
                        obj.totalAvgLevelActivity = totalAvgLevelActivity / counter;
                        arrayReports.push(obj);
                        // console.log('not yet finished.');
                    }
                }
            );
        });

        res.json(arrayReports);
        console.log('finished.');

任何人都知道如何在 Node.js 中实现这一目标

Answer 1

mongoose 已被承诺，您不必使用async 来处理流程，也不必为简单的 forEach 使用 lodash。而且您通过 _id 请求查找是没用的，您已经有一个 Student 对象：

Student.find({ status: 'student' })
    // .populate('student') // why this?
    .then(function (students) {
        // build an array of promises
        var promises = students.map(function (student) {
            return WorksnapsTimeEntry.find({
                "student": student._id;
            });
        });

        // return a promise to continue the chain
        return Promise.all(promises);
    }).then(function(results) {
        // results are the matching entries
        console.log('third');
        var totalMinutes = 0;
        var totalAvgLevelActivity = 0;
        var counter = 0;
        _.forEach(results, function (item) {
            _.forEach(item.timeEntries, function (item) {
                if (item.duration_in_minutes) {
                    totalMinutes = totalMinutes + parseFloat(item.duration_in_minutes[0]);
                }

                if (item.activity_level) {
                    totalAvgLevelActivity = totalAvgLevelActivity + parseFloat(item.activity_level[0]);
                    counter++;
                }
            });
        });

        var obj = {};
        obj.studentId = 'test';
        obj.firstName = 'test';
        obj.lastName = 'test';
        obj.municipality = 'test';
        obj.totalMinutes = totalMinutes;
        obj.totalAvgLevelActivity = totalAvgLevelActivity / counter;
        arrayReports.push(obj);
        // console.log('not yet finished.');
        res.json(arrayReports);
        console.log('finished.');
    }).catch(function(err) {
        return res.status(400).send({
            message: errorHandler.getErrorMessage(err)
        });
    });