在Mongoose查询中使用JavaScript Promise

Question

Normally if I was going to run multiple mongoose queries I would use the built in promise to chain them all together. In this case, the user chooses which schemas to search. This could be one of them or both. The following example uses the post data to define which schemas to search and if one is false, it should continue through the promise chain. Right now the final promise is being called before the queries.

Example in my express controller:

app.post('/custom-search', function (req, res) {
     var single = false
     var multi = false


      if(req.body.single){
          var single = true
      }

      if(req.body.multi){
          var multi = true
      }

    var promise = new Promise(function (resolve, reject) {
        if(multi){
            multiSchema.find({}, function (err, result) {
                if(!err){
                    console.log(result);
                    resolve()
                }
            })
        }else{
            resolve()
        }
    }).then(function (value) {
        if(single){
            singleSchema.find({}, function (err, result) {
                if(!err){
                    console.log(result);
                    resolve()
                }
            })
        }else{
            resolve()
        }
    }).then(function (value) {
        console.log("done");
    })
})

});

output:

>done
>[singleResults]
>[multiResults]

done should be printing last so that is the first problem.

Answer 1

Like we discussed, few things had to be clean up. First by actually using and returning the promise for it work properly, Second, creating a mini-promise within your first .then() to resolve and reject your single conditional statement. And third, handling/catching promises.

I wrote a pseudo version of your code to illustrate my point of view, hopefully it may be of a good use.

app.get('/custom-search', function (req, res) {
        // Manipulating values to test responses
        var single = false;
        var multi = true;

        var promise = new Promise(function (resolve, reject) {
            if (multi) {
                setTimeout(function () {
                    resolve('MULTI RESOLVED!');
                }, 3000);
            } else {
                reject('MULTI REJECTED!');
            }
        })
            .then(function (value) {
                new Promise(function (resolve, reject) {
                    if (single) {
                        setTimeout(function () {
                            resolve('SINGLE RESOLVED!');
                        }, 3000);
                    } else {
                        reject('SINGLE REJECTED!');
                    }
                })
                    .catch(function (error) {
                        console.log('SINGLE ERROR!', error);
                    })
                    .then(function (value) {
                        console.log('SINGLE DONE', value);
                    });
            })
            .catch(function (error) {
                console.log('MULTI ERROR!', error);
            });
        return promise;
    });