我如何使用Promise（Q package）实现此目的？

Question

I am developing an app with express.js and mongodb. I want to do following task. If database is available then i want to fetch all posts else it will throw an error.. I am using Q package for promise. I am not sure how can i make the following code work.. If there is any other way to achieve this then help me out.

function getAllPosts(){

    var deferred = Q.defer();

    db.connectDB().then(function(){ 

    db.posts.find({},function(err,data){

        if(err){
            deferred.reject(new Error('problem'));
        }else{
            deferred.resolve(data);
        }
    });

    }).fail(function(err){
        console.log(err);
    });

    return deferred.promise;
}

Here is my code for route localhost:3000/posts

exports.posts = function(req,res){
  postModel.getAllPosts().then(function(data){
    res.send(data);
  }).fail(function(err){
    res.send('error');
  });
}

Here is my mongodb connection code

function connectDB(){
    mongoose.connect('mongodb://localhost/demo');
        var deferred = q.defer();
    mongoose.connection.on('open',function(){
        createTables();
        deferred.resolve();
    });

    mongoose.connection.on('error',function(){

        deferred.reject(new Error('Error connecting database'));
    });

    return deferred.promise;
}

Answer 1

I think the problem is in your connection method, because you are not instantiating the deferred object with capital letter Q .

You should use

var deferred = Q.defer();

instead of

var deferred = q.defer();

So your connection method should look like:

function connectDB(){
    mongoose.connect('mongodb://localhost/demo');
    var deferred = Q.defer(); // Q with capital letter
    mongoose.connection.on('open',function(){
        createTables();
        deferred.resolve();
    });

    mongoose.connection.on('error',function(){

        deferred.reject(new Error('Error connecting database'));
    });

    return deferred.promise;
}