如何格式化 mysql 查询以利用异步/承诺？

Question

我正在尝试使用 2 个参数对我的数据库进行简单查询，但在尝试使用 Promises/Async 重新格式化此代码以避免回调地狱时遇到了麻烦。代码如下：

module.exports = {
  getDailyRaw: function(school, meal, callback) {
     var sql = "SELECT * FROM daily WHERE school = ? AND meal = ?";
     pool.getConnection(function(err, conn) {
         if(err) { console.log(err); callback(true); return; }
         conn.query(sql, [school, meal], function(err, results) {
             conn.release();
             if(err) { console.log(err); callback(true); return; }
             callback(false, school, results);
         })
     })
  }, ....

我一直在尝试查找有关解决此问题的教程，但无法正确实施/理解它们。

编辑：我现在正在使用“promise-mysql”库来完成工作，但我仍然无法将多个查询链接在一起。我如何将已经建立的连接传递给另一个 then 语句，以便我可以再次调用查询？ 更新代码：

getDaily: function(school, meal, callback) {
    pool.getConnection().then(function(conn) {
        var sql = "SELECT * FROM daily WHERE school = ? AND meal = ?";
        return conn.query(sql, [school,meal]);
    }).then(function(rows) {
        var qMarks = "";
        var foodNames = [school];
        rows.forEach(function(item) {
            foodNames.push(item.name);
            qMarks += "?,";
        });
        qMarks = qMarks.substring(0, qMarks.length - 1);
        var sql = "SELECT * FROM foods WHERE school = ? AND name IN (" + qMarks + ")";
        return conn.query(sql, foodNames);
    }).then(function(finalData) {
        callback(finalData);
    }).catch(function(err) {
        callback(null);
        console.log(err);
    })
},

Answer 1

您可以通过同步执行器nsynjs 以顺序方式编写和执行您的逻辑。它逐步运行代码，自动暂停承诺并等待它们的解决，然后再进行下一步。代码将如下所示：

function synchronousCode(pool, school, meal){
    var conn = pool.getConnection().data;
    var sql = "SELECT * FROM daily WHERE school = ? AND meal = ?";
    var rows = conn.query(sql, [school,meal]).data;
    // rows from 'daily' table are ready at this point
    var qMarks = "";
    var foodNames = [school];
    for(var i=0; i<rows.length; i++) {
        foodNames.push(rows[i].name);
        qMarks += "?,";
    };
    qMarks = qMarks.substring(0, qMarks.length - 1);
    sql = "SELECT * FROM foods WHERE school = ? AND name IN (" + qMarks + ")";
    var foods = conn.query(sql, foodNames).data;
    // rows from 'foods' table are ready at this point
    ... process food, or ...
    return foods;
};

var nsynjs = require('nsynjs');
var school =..., meal = ..., pool = ...;
nsynjs.run(synchronousCode,{},pool, school, meal,function(foods){
    console.log('synchronousCodeis done',foods);
});

Answer 2

有多种方法可以使来自一个链式.then() 的变量在链中的某个位置可用。您可以在How to Chain and Share Prior Results with Promises 中查看这些选项的摘要。

对于这种特殊情况，我建议像这样进行一层嵌套：

getDaily: function(school, meal, callback) {
    pool.getConnection().then(function(conn) {
        var sql = "SELECT * FROM daily WHERE school = ? AND meal = ?";
        return conn.query(sql, [school,meal]).then(function(rows) {
            var qMarks = "";
            var foodNames = [school];
            rows.forEach(function(item) {
                foodNames.push(item.name);
                qMarks += "?,";
            });
            qMarks = qMarks.substring(0, qMarks.length - 1);
            var sql = "SELECT * FROM foods WHERE school = ? AND name IN (" + qMarks + ")";
            return conn.query(sql, foodNames);
        }).then(function(finalData) {
            callback(finalData);
        });
    }).catch(function(err) {
            callback(null);
            console.log(err);
    });
},

而且，最好完全摆脱回调，只返回承诺并让调用者使用承诺。然后，如果调用者想要与其他人同步这个异步操作，它已经使用了一个承诺让他们更容易：

getDaily: function(school, meal) {
    return pool.getConnection().then(function(conn) {
        var sql = "SELECT * FROM daily WHERE school = ? AND meal = ?";
        return conn.query(sql, [school,meal]).then(function(rows) {
            var qMarks = "";
            var foodNames = [school];
            rows.forEach(function(item) {
                foodNames.push(item.name);
                qMarks += "?,";
            });
            qMarks = qMarks.substring(0, qMarks.length - 1);
            var sql = "SELECT * FROM foods WHERE school = ? AND name IN (" + qMarks + ")";
            return conn.query(sql, foodNames);
        });
    }).catch(function(err) {
        console.log(err);
        throw err;
    });
},

注意：如果您要返回 promise 并希望在 .catch() 中记录错误，则必须重新抛出错误以确保返回的 promise 仍然被拒绝（就像 try/catch 一样）。