Question

我是Node js的新手，我正在尝试最近几天实现此功能，但我无法修复

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我还有一个查询？在节点js中：如果表具有用户，则用户具有不同的关系表，如订单，配置文件，地址

如何实现此

首先我得到了用户用户循环
获取每个用户的个人资料，地址，订单
最终用户循环

但是上面的场景我无法在节点js中实现，但在php中非常简单，就像这样

exports.get_exercises_for_trainer = function(req, res)
    {

                connection.query('SELECT * FROM ag_exercise', function(err, exercise)
                 {


                    console.log('------------------------------before add fields ----------------------------------');
                    console.log(exercise);

                    for (var i in exercise)
                  {

                        fields(exercise[i].wt_id, function(result1) {


                                    exercise[i].wt_fields = result1; //here i am adding result set of other query but i am not geting this fields data

                                    console.log(result1) //but i printed here working fine but i need this result1 data out side query 

                          });

                    }

                    console.log('------------------------------after add fields ----------------------------------');
                    console.log(exercise);
                    res.render('pages/trainer_home.ejs',{page_title:"Exercise Create",exercise:exercise});

                    });


          }

        function fields(wt_id,callback) 
        {
            connection.query('SELECT *  FROM ag_workout_type_fields  WHERE wt_id = "'+wt_id+'"', function( err1, result1){

                  callback(result1);

             });

             }

Answer 1

这里有三个主要问题，我将分别加分。

问题1：在制作异步功能时，如何在该功能之外访问我的数据？

来自异步调用的所有数据都是通过回调，事件监听器或promises（一个花哨的回调和事件监听器处理程序）来访问的。在大多数情况下，您将只使用回调。所以，而不是：

get_user = function(user_id){
    //Do some stuff to get the user
    return the_user;
};
var user = get_user('1234');
//do whatever you want with user

你会看到：

get_user = function(user_id,callback){
    //Do some stuff to get the user
    callback(null,the_user);
}
get_user('1234',function(err,user){
    //do whatever you want with user
});

当我们进入问题3时，您将看到更复杂的用例。

问题2：如何循环访问我的数据，对每一行执行子查询，并将该数据附加到我当前的数据？

这里有几个问题。

每次查询数据库时，都在执行异步功能，因此需要相应地管理所有这些回调。幸运的是，有一些很棒的工具可以帮助你，我们将使用异步。
每次在for循环中调用异步函数时，for循环都会继续，因此你的迭代器会被覆盖，但你的异步函数还没有完成，所以你会得到各种意想不到的结果，比如消失变量，或错过映射的结果。您可以使用JavaScript闭包来处理这个问题，或者，您可以再次依赖像async这样的库来处理它。

我们不会在查询结果上运行for循环，而是将其传递给async.forEachOf，我们将使用它来修改现有数组，并将后续查询的结果附加到主查询的行。请务必注意，forEachOf将并行运行后续查询，因此您不应使用单个数据库连接，而应使用池。如果必须使用单个数据库连接，请改用forEachOfSeries。

async = require('async');
exports.get_exercises_for_trainer = function(req, res){
    connection.query('SELECT * FROM ag_exercise', function(err, exercises)
    {
        console.log('------------------------------before add fields ----------------------------------');
        console.log(exercises);
        async.forEachOf(exercises,function(exercise,index,callback){
            connection.query('SELECT * FROM ag_workout_type_fields WHERE wt_id = "' + exercise.wt_id + '"', function( err, result1){
                if(err)return callback(err1);
                exercises[index].wt_fields = result1; //Modify original array
                return callback();
            });
        },function(err){
            if(err){return;} //do some error handling
            console.log('------------------------------after add fields ----------------------------------');
            console.log(exercises);
            res.render('pages/trainer_home.ejs',{page_title:"Exercise Create",exercise:exercises});
        });
    });
};

问题3：如何执行许多相关但不同的查询，以便我可以填充有关我的对象的信息？

这是异步库的另一个很好的用法。在这种情况下，由于查询都不同，我们将使用parallel而不是forEachOf。

async = require('async');
populate_user = function(user,_callback){
    async.paralell({
        profile: function(callback){
            var sql = "SELECT * FROM profiles WHERE user_id = " + user.id + " LIMIT 1 ";
            var connection.query(sql,function(err,rows,fields){
                if(err)return callback(err);
                if(rows.length === 1)return callback(null,rows[0]);
                return callback(null,[]);
            });
        },
        address: function(callback){
            var sql = "SELECT * FROM addresses WHERE user_id = " + user.id + " LIMIT 1 ";
            var connection.query(sql,function(err,rows,fields){
                if(err)return callback(err);
                if(rows.length === 1)return callback(null,rows[0]);
                return callback(null,[]);
            });
        },
        orders: function(callback){
            var sql = "SELECT * FROM orders WHERE user_id = " + user.id;
            var connection.query(sql,function(err,rows,fields){
                if(err)return callback(err);
                if(rows.length > 0)return callback(null,rows); //notice how this one could have multiple results so we're returning them all
                return callback(null,[]);
            });
        },
    },
    function(err,result){
        if(err)return _callback(err);
        for(var att in result){user[att] = result[att];}
        callback(null,user);
    }
}
user = {id:1234};
populate_user(user,function(err,populated_user)){
    console.log(user); //wow notice how it's populated too!
    console.log(populated_user); //this is really just a new handle for the same object
});

我想要注意的是，没有对此进行测试，甚至没有测试语法，因此可能需要稍微改动。

Node JS如何获取外部的查询数据

1 个答案: