在 node+express 中使用 sequelize 从数据库进行顺序查询

Question

我刚刚开始节点并且来自 PHP 背景。一段时间以来，我一直试图弄清楚 Promise，但无法弄清楚。

我试图在循环中的第二个查询中使用一个查询的结果，但有两个问题：

-ONE：typeof 值显示一个对象；但是当我安慰它时，它显示未定义

-二：我的结果没有按正确的顺序排列。我正在尝试使用 'then()' 但无法完全弄清楚。 “inside promise 2”在 'then()' 1 块中的循环之前被安慰

这是我在代码中所做的：

exports.followers_temp = function(req, res)
{
    db.sequelize.query('SELECT DISTINCT public."Users".id as user_id, public."Users".full_name, public."Users".profile_image_url, (SELECT COUNT(*) FROM public."Followships" WHERE public."Followships".leader_id = public."Users".id AND public."Followships".follower_id = :leader_id) AS is_followed FROM public."Users"  INNER JOIN public."Followships" ON public."Users".id = public."Followships".follower_id WHERE  public."Followships".leader_id = :leader_id LIMIT :limit OFFSET :offset',  { replacements: { leader_id: req.params.leader_id, limit: req.params.limit, offset: req.params.offset }, type: db.sequelize.QueryTypes.SELECT}).
    then(function(data){
      console.log('i am here; length: ' + data.length);
        return new Promise(function(resolve, reject) {
              console.log('inside promise 1, should have access to data object');

              console.log('before loop');
              for(let i=0; i < data.length; i++){
                  var temp = db.sequelize.query('(SELECT COUNT(*) AS is_following FROM public."Followships" WHERE public."Followships".leader_id = ' + data[i].user_id + ' AND public."Followships".follower_id = :my_id)',  { replacements: { leader_id: req.params.leader_id, my_id: req.params.my_id, limit: req.params.limit, offset: req.params.offset }, type: db.sequelize.QueryTypes.SELECT})
                  temp.then(function(value){

                    // Issue ONE
                      console.log('type: '+ typeof value); // it's an object
                      console.log('is_following: '+ value.is_following); // yet it's giving undefined

                      console.log('value: '+ JSON.stringify(value)); // but it prints out when I stringify it

                      data[i].is_followed = value.is_following;
                  });
              }
            resolve(data);
            });
    })

    .then(function(my_array){
        // Issue TWO
        console.log('inside promise 2');

        return res.status(200).send({
            error:false,
            message: my_array
        });
    })
        .catch(function(err) {
            console.log('inside promise catch');
            return res.status(400).send({
                error:true,
                message: errorHandler.getErrorMessage(err)
            });
        });
};

这是控制台输出：

i am here; length: 5          
inside promise 1, should have 
before loop                   
inside promise 2              
type: object                  
is_following: undefined       
value: [{"is_following":"1"}] 
type: object                  
is_following: undefined       
value: [{"is_following":"0"}] 
type: object                  
is_following: undefined       
value: [{"is_following":"1"}] 
type: object                  
is_following: undefined       
value: [{"is_following":"1"}] 
type: object                  
is_following: undefined       
value: [{"is_following":"0"}] 

请告诉我 -我究竟做错了什么？ - 如果你能明智地使用 Promise 语法......那就太好了

感谢您的期待；请忽略菜鸟的错误，因为我现在是菜鸟。

Answer 1

在更深入地搜索 stackoverflow 之后，我能够解决我的问题。我使用了 THIS POST 上提供的帮助。

感谢 stackoverflow 社区提供的所有帮助。今后我也会努力做出贡献的。

最终解决方案如下所示：

    exports.followers_temp = function (req, res) {
    db.sequelize.query('SELECT DISTINCT public."Users".id as user_id, public."Users".full_name, public."Users".profile_image_url, (SELECT COUNT(*) FROM public."Followships" WHERE public."Followships".leader_id = public."Users".id AND public."Followships".follower_id = :leader_id) AS is_followed FROM public."Users"  INNER JOIN public."Followships" ON public."Users".id = public."Followships".follower_id WHERE  public."Followships".leader_id = :leader_id LIMIT :limit OFFSET :offset', { replacements: { leader_id: req.params.leader_id, limit: req.params.limit, offset: req.params.offset }, type: db.sequelize.QueryTypes.SELECT })
        .then(function (users) {
            var promises = users.map(function(user){
                return db.sequelize.query(
                    '(SELECT COUNT(*) AS is_following FROM public."Followships" WHERE public."Followships".leader_id = ' +
                    user.user_id + ' AND public."Followships".follower_id = :my_id)',
                    { replacements:
                            { leader_id: req.params.leader_id,
                                my_id: req.params.my_id,
                                limit: req.params.limit,
                                offset: req.params.offset
                            },
                        type: db.sequelize.QueryTypes.SELECT
                    })
                    .then(function(results){
                        user.is_followed = results[0].is_following;
                        return user
                    })
            })
            Promise.all(promises).then(function(results) {
                return res.status(200).send({
                    error: false,
                    message: users
                });
            })
                .catch(function (err) {
                    return res.status(400).send({
                        error: true,
                        message: errorHandler.getErrorMessage(err)
                    });
                });
        })
    .catch(function (err) {
        return res.status(400).send({
            error: true,
            message: errorHandler.getErrorMessage(err)
        });
    });
};

Answer 2

如果你已经有一个promise，你不需要使用new Promise：

exports.followers_temp = function (req, res) {
  db.sequelize.query('SELECT DISTINCT public."Users".id as user_id, public."Users".full_name, public."Users".profile_image_url, (SELECT COUNT(*) FROM public."Followships" WHERE public."Followships".leader_id = public."Users".id AND public."Followships".follower_id = :leader_id) AS is_followed FROM public."Users"  INNER JOIN public."Followships" ON public."Users".id = public."Followships".follower_id WHERE  public."Followships".leader_id = :leader_id LIMIT :limit OFFSET :offset', { replacements: { leader_id: req.params.leader_id, limit: req.params.limit, offset: req.params.offset }, type: db.sequelize.QueryTypes.SELECT })
  .then(function (users) {
    console.log('i am here; length: ' + users.length);
    //do not use new Promise if you already have a promie
    //https://stackoverflow.com/a/48576246/1641941
    //use promise.all and map data to promises
    return Promise.all(
      users.map(
        user=>{
          db.sequelize.query(
            '(SELECT COUNT(*) AS is_following FROM public."Followships" WHERE public."Followships".leader_id = :leader_id AND public."Followships".follower_id = :my_id)', 
            { replacements: 
              { leader_id: user.user_id,//changed leader_id
                my_id: req.params.my_id//removed limit and offset 
              },
              type: db.sequelize.QueryTypes.SELECT 
            })
            .then(function (value) {
              console.log('type: ' + typeof value);
              //value is an array
              console.log('is_following: ' + value[0].is_following);
              console.log('value: ' + JSON.stringify(value));    
              user.is_followed = value[0].is_following;
              return user;
            });
        }
      )
    )
  })
  .then(function (users) {
    // Issue TWO
    console.log('inside promise 2',users);
    return res.status(200).send({
      error: false,
      message: users
    });
  })
  .catch(function (err) {
    console.log('inside promise catch',err);
    return res.status(400).send({
      error: true,
      message: errorHandler.getErrorMessage(err)
    });
  });
};

Answer 3

db.sequelize.query 是放在for 循环中的异步调用。这将启动查询，但尚未成功完成。因此，您的 data 尚未完成，您已经在解决它。使用Promise.all 可以这样写：

Promise.all(data.map((val) => { // here val = data[i] inside the query
    return new Promise((resolve, reject) => {
        db.sequelize.query(...)
            .then((value) => {
                // modify your data object here.
                resolve();
            });
    });
})).then(() => { resolve(data);});