mongoose exec call with async.parallel - node.js

Question

I have a code like this in my express controller
function (req, res) {
var queries = [];
data.forEach(function (item) {
var query = myModel.findOneAndUpdate({remoteId: item.id}, item, {upsert: true}).exec;
queries.push(query);
});
async.parallel(queries, function (err, docs) {
res.json(docs);
});
});
If data array has 3 item, then i have an array of 3 null values.
async.parallel function accepts a function with a callback argument, that should be called to properly complete its execution. So mongoose.Query.exec does the same. But i recieve an array of null objects as a result.
If i wrap my exec call like so
var query = function (cb) {
tournamentsModel.findOneAndUpdate({remoteId: item.id}, item, {upsert: true}).exec(function (err, model) {
cb(err, model);
});
};
queries.push(query);
everything is ok and i recieve 3 docs from mongo as a result.
Why should i explicitly call a callback passed to a async.parallel function call, when exec method does the same?

Answer 1

When you directly pass the exec function of your query to async.parallel as a function to execute, you're losing the this context of that function call that contains your query to run.
To use this approach, you would need to call bind to return a new function that will call exec with the right this context; so something like this:
var query = Query.prototype.exec.bind(
myModel.findOneAndUpdate({remoteId: item.id}, item, {upsert: true})
);
queries.push(query);
It's probably cleaner to call exec yourself, but pass in the async callback to it:
var query = function(cb) {
myModel.findOneAndUpdate({remoteId: item.id}, item, {upsert: true}).exec(cb);
}
queries.push(query);