Mongoose returns empty while the same query in mongodb shell works fine

I know maybe this question has been asked quite many times here, I've went through several solutions people came with to similar questions but none of them seemed to help in my case.

I have two collections called users and posts and models for them look like this:

users

var mongoose = require('mongoose').set('debug', true);
var Schema = mongoose.Schema;

var usersSchema = new Schema({
    name: {type: String, required: true}
});

var User = mongoose.model('user', usersSchema, 'users');

module.exports = User;

posts

var mongoose = require('mongoose').set('debug', true);
var Schema = mongoose.Schema;

var postsSchema = new Schema({
    content: String,
    user: {
        type: Schema.ObjectId,
        ref: 'users',
        required: true
    }
});

var Post = mongoose.model('post', postsSchema, 'posts');

module.exports = Post;

I'm trying to get the posts of a user using this code:

var Post = require('../models/posts');

...

router.get('/posts/user/:userId', function (req, res, next) {
    Post.find({user: req.params.userId}, function (err, posts) {
        Post.populate(posts, {path: 'user'}, function(err, posts) {
            res.send(posts);
        });
    });
});

Mongoose debug mode reports that the following query is executed during the request:

posts.find({ user: ObjectId("592e65765ba8a1f70c1eb0bd") }, { fields: {} })

which works perfectly fine in mongodb shell (I'm using Mongoclient) but with Mongoose this query returns an empty array.

The query I run in mongodb shell:

db.posts.find({ user: "592e65765ba8a1f70c1eb0bd" })

The results I get:

{ "_id" : ObjectId("592e66b48f60c03c1ee06445"), "content" : "Test post 3", "user" : "592e65765ba8a1f70c1eb0bd" }
{ "_id" : ObjectId("592e66b98f60c03c1ee06446"), "content" : "Test post 4", "user" : "592e65765ba8a1f70c1eb0bd" }
{ "_id" : ObjectId("592e66bb8f60c03c1ee06447"), "content" : "Test post 5", "user" : "592e65765ba8a1f70c1eb0bd" }

I'm at the very beginning on learning Node.JS and MongoDB, so maybe I've missed something.

Thank you in advance!

via Eseth