Mongodb : find documents including references to sub-documents [node-mongodb-native] - mongodb

Question

I have the following Mongodb Documents :
Users : {'_id' : '12345', 'fullname' : 'ABC'}
and
Files :
{'_id' : 'File001', 'path' : 'pathA', 'Users_id' : '12345'}
{'_id' : 'File002', 'path' : 'pathB', 'Users_id' : '12345'}
How do i query or find all 'Files' documents in a way that the 'Users_id' which is referencing to the 'Users' document has the full 'Users' Document?
Expected Output :
{'_id' : 'File001', 'path' : 'pathA', 'Users_id' : '12345', 'user' : {'_id' : '12345', 'fullname' : 'ABC'}}
{'_id' : 'File002', 'path' : 'pathB', 'Users_id' : '12345', 'user' : {'_id' : '12345', 'fullname' : 'ABC'}}
In this way, i could access the file owner's fullname as such : file.user.fullname
I Appreciate any help from you guys. Thank you.
--- EDIT
I am using node-mongodb-native to access the db.
Below is my code to retrieve it:
var files = db.collection ('Files');
files.find ({}, {}).toArray (function ( err, filesDoc){
for ( var index in filesDoc) {
var fileDoc = filesDoc [ index ];
var users = db.collection ('Users');
users.findOne ({'_id' : fileDoc.Users_id}, {}, function (errUser, userDoc){
if ( ! errUser ) {
fileDoc.user = userDoc;
}
});
}
});
but this code is not assigning the user to all files doc and only assigns to the last element of the filesDoc array. Any suggestions?

Answer 1

MongoDB doesn't support joins so you have to query for the User details separately. If you're using node.js, Mongoose provides a populate feature to help simplify this so you can do something like the following to pull in the user details:
Files.find().populate('Users_id')

Answer 2

I managed to find the cause of the issue. The reason why my code returns a null user is because I saved File.Users_id as String and I am retrieving it using ObjectId.
I fixed my codes by converting Users_id String to ObjectId before saving the File.