Can't remove headers after they are sent in Koa - node.js

Question

I coded a minimalist server to manage REST services. The only coded route is supposed to retrieve data from mongo, then to send it to the client. Code is below.
var kr = require('koa-route');
var koa = require('koa');
var app = koa();
var MongoClient = require("mongodb").MongoClient;
var events = function * ()
{
var _this = this;
MongoClient.connect("mongodb://localhost/eventdata", function(error, db) {
if (error) throw(error);
console.log("Connecté à la base de données");
db.collection('events').find().toArray(function(err, array)
{
console.log('returning %d objects', array.length);
db.close();
_this.body = {eventsArray : array};
});
});
}
app.use(kr.get('/events', events));
app.listen(3000);
Koa is version 1.2.4
When I try to reach it, I get the error message "Can't remove headers after they are sent". I managed to understand that the program (sort of) finishes to send back response before the find() is over. So when the _this.body = { ... } is called, this causes the error.
Now question is : how to fix that ?
I just began to practice Koa, so there are a lot of stuff I'm not used to.
I tested solution proposed here : Can't remove headers after they are sent but it doesn't work. I get the following error message :
eventsArray = yield db.collection('events').find({})
^^
SyntaxError : unexpected identifier
Thanks in advance for your help.

Answer 1

it looks like you need to wrap db req to promise. Try in this way
var kr = require('koa-route');
var koa = require('koa');
var app = koa();
var MongoClient = require("mongodb").MongoClient;
var eventsPromise = function(){
return new Promise(function(resolve, reject){
MongoClient.connect("mongodb://localhost/eventdata", function(error, db) {
if (error){
return reject(error);
}
console.log("Connecté à la base de données");
db.collection('events').find().toArray(function(err, array)
{
console.log('returning %d objects', array.length);
db.close();
return reslove({eventsArray : array});
});
});
});
}
var events = function * ()
{
var event = yiled eventsPromise();
this.body = event;
}
app.use(kr.get('/events', events));
app.listen(3000);