I am trying to join two collections and being able to get the combined data. To do so using Mongoose, i am supposed to use the populate syntax to achieve that. I am receiving error that the Schema Schema hasn't been registered for 'User_Fb'. From my code, I have exported the models and required in my server.js but the error is still appearing. What have I done wrong?

feed_post.model.js

var mongoose = require('mongoose');
var conn_new_app     = mongoose.createConnection('mongodb://localhost/new_app');
var User_fb = require('../models/fb_db.model');

var Schema = mongoose.Schema;
var feed_postSchema = new Schema({
    user_id:  { type: Schema.Types.ObjectId, ref: 'User_Fb' },
    content: String,
    location: String,
    image: [{ type : String }]
});

var Feed_Post = conn_new_app.model('Feed_Post', feed_postSchema);

module.exports = Feed_Post;

fb_db.model.js

var mongoose = require('mongoose');
var conn_new_app     = mongoose.createConnection('mongodb://localhost/new_app');
var Feed_Post = require('../models/feed_post.model');

var user_fb = new mongoose.Schema({
    name: String,
    location: String,
    fb_id: Number
});

var User_Fb = conn_new_app.model('User_Fb', user_fb);

module.exports = User_Fb;

server.js

var express = require('express'),

mongoose = require('mongoose'),
User = require('./app/models/user.model'),
Post = require('./app/models/post.model'),
Maptest = require('./app/models/maptest.model'),
Feed_Post = require('./app/models/feed_post.model'),
User_Fb = require('./app/models/fb_db.model'),

app = express();

app.get('/testget', function(req,res){
    Feed_Post.findOne().populate('user_id').exec(function(err, c) {
        if (err) { return console.log(err); }

        console.log(c.fk_user.userName);
    });
});

UPDATED from Pier-Luc Gendreau Answer's

fb_db.model.js

module.exports = function (connection) {
    var mongoose = require('mongoose');
    var Schema = mongoose.Schema;

    var user_fb = new mongoose.Schema({
        name: String,
        location: String,
        fb_id: Number
    });

    return connection.model('User_FB', user_fb);;
}

feed_post.model.js

module.exports = function (connection) {
    var mongoose = require('mongoose');
    var Schema = mongoose.Schema;

    var feed_postSchema = new Schema({
        user_id:  { type: Schema.Types.ObjectId, ref: 'User_Fb' },
        content: String,
        location: String,
        image: [{ type : String }]
    });

    return connection.model('Feed_Post', feed_postSchema);;
}

server.js

var express = require('express'),
app = express(),

mongoose = require('mongoose'),
conn_new_app = mongoose.createConnection('mongodb://localhost/new_app'),
User_Fb = require('./app/models/fb_db.model')(conn_new_app),
Feed_Post = require('./app/models/feed_post.model')(conn_new_app);

app.get('/testget', function(req,res){
    Feed_Post.find().populate('user_id').exec(function(err, res) {
        if (err) { return console.log(err); }
        console.log(res);
    });
});

The problem is that you are creating a new connection in each and every model, so you end up with a bunch of different connection objects. Even though they are pointing to the same database, mongoose models don't know about other connections. You should instead create the connection object in your main app and then pass it around.

server.js

var express = require('express');
var mongoose = require('mongoose');

var app = express();
var conn_new_app = mongoose.createConnection('mongodb://localhost/new_app');
var Feed_Post = require('./app/models/feed_post.model')(conn_new_app);

app.get('/testget', function(req,res){
  Feed_Post.findOne().populate('user_id').exec(function(err, c) {
    if (err) { return console.log(err); }
    console.log(c.fk_user.userName);
  });
});

Then modify your models so they are actually a function:

feed_post.model.js

module.exports = function (connection) {
  var mongoose = require('mongoose');
  var Schema = mongoose.Schema;

  var feed_postSchema = new Schema({
    user_id:  { type: Schema.Types.ObjectId, ref: 'User_Fb' },
    content: String,
    location: String,
    image: [{ type : String }]
  });

  return connection.model('Feed_Post', feed_postSchema);;
}

Also, like Adrian mentions, you can use mongoose as a singleton. However, I do recommend using createConnection as you did because if you ever need a second connection, you won't have to refactor the connection code to your first database.