Promisify cursor execution: MongoDB Native Driver

Question

I already read up on these, but I'm having a bit of trouble executing it. mongoDB promise gets returned too early

Specifically, I'm trying to promisify the collection.find cursor, but I'm not sure I'm doing it right... any help would be appreciated:

//mongo.js

var promise = require('bluebird');
var mongodb = require('mongodb');
var mongoClient = mongodb.MongoClient;
var collection = mongodb.Collection;
promise.promisifyAll(mongodb);
promise.promisifyAll(collection.prototype);

collection.prototype._find = collection.prototype.find;
collection.prototype.find = function() {
  var cursor = this._find.apply(this, arguments);
  cursor.toArrayAsync = promise.promisify(cursor.toArray, cursor);
 return cursor;
};


var myfunction = function(callback){
  // configure and export database (async)
  mongoClient.connectAsync(database)
  .then(function(db){
    module.exports.db = db;
    promise.all([
      db.createCollectionAsync('hoth', {w:1})
        .then(function(collection){
          module.exports.hoth = collection;
          collection.ensureIndexAsync({time:-1, location:'2dsphere'})
          .done(function(index){console.log(index+' hoth');});
        }),
      db.createCollectionAsync('endor', {w:1})
        .then(function(collection){
          module.exports.endor = collection;
          collection.ensureIndexAsync({time:-1, location:'2dsphere'})
          .done(function(index){console.log(index+' endor');});
        }),
      db.createCollectionAsync('alderaan', {w:1})
        .then(function(collection){
          module.exports.alderaan = collection;
          collection.ensureIndexAsync({time:-1, location:'2dsphere'})
          .done(function(index){console.log(index+' alderaan');});
        })
    ])
    .done(function(){callback(null);});
  });
};
module.exports.myfunctionAsync = promise.promisify(myfunction);

and here's app.js

// app.js
mongo.myfunctionAsync()
.then(function(result){
  mongo.hoth.mapReduceAsync(u.map, u.reduce,{
  out: {replace:'deathstar'}, scope:{size:0.01},
  query: {time:{$gte:ago}}, finalize:u.finalize});
}).then(function(deathstar){
  deathstar.findAsync({},{fields:{_id:0, value:1}});
}).then(function(docs){
  var final = _.map(docs, function(doc){return doc.value;});
}).then(function(final){
  mongo.alderaan.insertAsync(final, {w:1});
}).then(console.log('done'));

and here's the error

Cannot call method 'findAsync' of undefined

Just so you know, promises work by chaining through _return values_ (just like sync code!) so you should `return mongo.hoth....` and `return deathstar.findAsync(...` etc — Benjamin Gruenbaum, Jul 21 '14 at 19:39

score 4 · Accepted Answer · answered Jul 09 '15 at 21:19

With bluebird 2.0 you can just promisify the npm. After that cursor is promisified too.

Here is a general example with a cursor:

var MongoClient = require('bluebird').promisifyAll(require("mongodb")).MongoClient;

Then:

MongoClient.connectAsync("mongodb://localhost:27017/something")
    .then(function(db) {
        db.collection('foo').findAsync({ })
            .then(function(cursor) {       
                return cursor.toArrayAsync();
            })
           .then(function(arrayOfFoos) {
               console.log(arrayOfFoos);
           });
    });

Promisify cursor execution: MongoDB Native Driver

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