Bug: TypeError: Cannot set property 'eachAsync' of undefine

[CarlosGRodriguezL]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
This just fails sometimes, when the model.collection.aggregate returns null or undefined.
In the decorateCursor the aggregate.js trys to add the eachAsync function, but as it is undefinde it crashes.

Aggregate.prototype.exec = function(callback) {
  ...
    var cursor = this._model.collection.
      aggregate(this._pipeline, this.options || {});
    decorateCursor(cursor);
    return cursor;
  ...
  return new Promise.ES6(function(resolve, reject) { ... });
};

function decorateCursor(cursor) {
  cursor.eachAsync = function(fn, callback) { ... };
}

What is the expected behavior?

It should wait till the aggregate is finished or check if the cursur is undefined and return a understandable exception.

Please mention your node.js, mongoose and MongoDB version.
mongoose version: 4.8.6
nodejs version: 6.9.x

[vkarpov15]

Confirmed. This issue will only happen if mongoose is not connected to mongodb.

Repro:

var Schema = mongoose.Schema;

mongoose.set('debug', true);

mongoose.connect('mongodb://localhost:27017/gh5145');

var schema = new Schema({});
var M = mongoose.model('Test', schema);

M.aggregate([{ $match: { name: 'test' } }]).cursor().exec().eachAsync(() => {}).then(res => console.log(res));

To work around this, wait for mongoose to connect:

var Schema = mongoose.Schema;

mongoose.set('debug', true);

mongoose.connect('mongodb://localhost:27017/gh5145').then(() => {
  var schema = new Schema({});
  var M = mongoose.model('Test', schema);

  M.aggregate([{ $match: { name: 'test' } }]).cursor().exec().eachAsync(() => {}).then(res => console.log(res));
});

Or use the async option for aggregation cursors:

var Schema = mongoose.Schema;

mongoose.set('debug', true);

mongoose.connect('mongodb://localhost:27017/gh5145');

var schema = new Schema({});
var M = mongoose.model('Test', schema);

M.aggregate([{ $match: { name: 'test' } }]).cursor({ async: true }).exec(function(error, cursor) {
  cursor.eachAsync(() => {}).then(res => console.log(res));
});

[vkarpov15]

In 4.10 we'll add a special flag useMongooseAggCursor that will let you do M.aggregate([{ $match: { name: 'test' } }]).cursor({ useMongooseAggCursor: true }).eachAsync() without waiting for mongoose to connect 👍