NodeJS + MongoJS: Nested Callbacks Issue - node.js

Question

I'm still a n00b with NodeJS so this question may be somewhat elementary.
I'm using MongoJS to read two logically related collections. The first find() returns a value that I pass to the second find() to get the information I need.
I've tried several strategies, the last one (snippet #1) being a class that I export.
Before that I just had a function that did a return, returning the desired value, i.e., "config[0]".
In this code all I did was set the "sapConfig" attribute to the word "test", but when I execute this code the value of "sapConfig" is always "null" after I call the "get_config()" method and - strangest of all - the reference to "this.sapConfig = 'test'" generates an error, i.e., "Cannot set property 'sapConfig' of undefined".
When I had the code just as a simple function with a return statement (snippet #2), no errors were generated but the value returned is always "undefined" although the console.log() statements show that the value of the variable being returned has the desired value. What gives?
Code Snippet #1: Returns Object
"use strict";
var mongojs = require('mongojs'); // MongoDB API wrapper
module.exports = function(regKey) {
this.regKey = regKey;
this.sapConfig = null;
this.get_config = function() {
// Read SAP connection information from our MONGO db
var db = mongojs('mongodb://localhost/MIM', ['Configurations','Registrations']);
db.Registrations.find({ key: this.regKey }, function(err1, registration){
console.log('Reg.find()');
console.log(registration[0]);
db.Configurations.find({ type: registration[0].type }, function(err2, config){
console.log('Config.find()');
console.log('config=' + config[0].user);
this.sapConfig = 'test';
});
});
}
this.get_result = function() {
return this.sapConfig;
}
}
Again, the code in snippet #1, when I make a call to "get_config()", results in an error when it executes the line "this.sapConfig = 'test'".
However, after this error I can execute "obj.get_result()" and I get the value to which it was initialized, i.e., null. In other words, that same code doesn't generate an error saying that the "this" is undefined as .in the "get_config()" method
Code Snippet #2: Using the "return" statement
"use strict";
var mongojs = require('mongojs'); // MongoDB API wrapper
module.exports = function(regKey) {
// Read SAP connection information from our MONGO db
var db = mongojs('mongodb://localhost/MIM', ['Configurations','Registrations']);
db.Registrations.find({ key: regKey }, function(err1, registration){
console.log('Reg.find()');
console.log(registration[0]);
db.Configurations.find({ type: registration[0].type }, function(err2, config){
console.log('Config.find()');
console.log('config=' + config[0].user);
return config[0].user;
});
});
}
When I receive the return value and inspect it, it's "undefined". For example, at the Node CL I issue the following commands:
var config = require('./config') // The name of the module above
> var k = config('2eac44bc-232d-4667-bd24-18e71879f18c')
undefined <-- this is from MongoJS; it's fine
> Reg.find() <-- debug statement in my function
{ _id: 589e2bf64b0e89f233da8fbb,
key: '2eac44bc-232d-4667-bd24-18e71879f18c',
type: 'TEST' }
Config.find()
config=MST0025
> k <-- this should have the value of "config[0]"
undefined
You can see that the queries were successful but the value of "k" is "undefined". What's going on here?
I don't care which approach I use I just need one of them to work.
Thanks in advance!

Answer 1

this.sapConfig is not accessible. Thats because this refers to within the current function. What I like todo, is have a variable that refers to the function instance that you know sapConfig is located in.
Ex:
function Foo() {
var self = this;
this.test = "I am test";
var bar = function(){
return function(){
console.log(this.test); //outputs undefined (because this refers to the current function scope)
console.log(self.test); //outputs "I am test";
}
}
}
Here is your first code snippit with my example implemented:
"use strict";
var mongojs = require('mongojs'); // MongoDB API wrapper
module.exports = function(regKey) {
var self = this;
this.regKey = regKey;
this.sapConfig = null;
this.get_config = function() {
// Read SAP connection information from our MONGO db
var db = mongojs('mongodb://localhost/MIM', ['Configurations', 'Registrations']);
db.Registrations.find({ key: this.regKey }, function(err1, registration) {
console.log('Reg.find()');
console.log(registration[0]);
db.Configurations.find({ type: registration[0].type }, function(err2, config) {
console.log('Config.find()');
console.log('config=' + config[0].user);
self.sapConfig = 'test';
});
});
}
this.get_result = function() {
return self.sapConfig;
}
}
For your second snippet. You are trying to return a value from within your nested callback. Since nested functions are asyncronous, you cannot do that.
Here is how I like to return values from nested callbacks:
Ex2:
//Function example
var functionWithNested = function(done) {
//Notice the done param.
// It is going to be a function that takes the finished data once all our nested functions are done.
function() {
//Do things
function() {
//do more things
done("resultHere"); //finished. pass back the result.
}();//end of 2nd nested function
}(); //end of 1st nested function
};
//Calling the function
functionWithNested(function(result) {
//Callback
console.log(result); //resultHere
})
Here is your code using that example:
"use strict";
var mongojs = require('mongojs'); // MongoDB API wrapper
module.exports = function(regKey, done) {
// Read SAP connection information from our MONGO db
var db = mongojs('mongodb://localhost/MIM', ['Configurations', 'Registrations']);
db.Registrations.find({ key: regKey }, function(err1, registration) {
console.log('Reg.find()');
console.log(registration[0]);
db.Configurations.find({ type: registration[0].type }, function(err2, config) {
console.log('Config.find()');
console.log('config=' + config[0].user);
done(config[0].user);
});
});
}
//Then wherever you call the above function use this format
// if config is the name of the function export above...
new Config().(regKey, function(result){
console.log(result); //config[0].user value
})
Lots and lots of code, but I hope you were able to follow it. Let me know if you have any more questions! Cheers.