MongoDB MapReduce - How to populate an array in reduce function?

I have a MovieRatings database with columns userId, movieId, movie-categoryId, reviewId, movieRating and reviewDate.

In my mapper I want to extract userId -> (movieId, movieRating)

And then in the reducer I want to group all movieId, movieRating pair by user.

Here is my attempt:

Map function:

var map = function() {
    var values={movieId : this.movieId, movieRating : this.movieRating};
    emit(this.userId, values);}

Reduce function:

var reduce = function(key,values) {
    var ratings = [];
    values.forEach(function(V){
        var temp = {movieId : V.movieId, movieRating : V.movieRating};
        Array.prototype.push.apply(ratings, temp);
        });
    return {userId : key, ratings : ratings };
}

Run MapReduce:

db.ratings.mapReduce(map, reduce, { out: "map_reduce_step1" })

Output: db.map_reduce_step1.find()

{ "_id" : 1, "value" : { "userId" : 1, "ratings" : [ ] } } 
{ "_id" : 2, "value" : { "userId" : 2, "ratings" : [ ] } } 
{ "_id" : 3, "value" : { "userId" : 3, "ratings" : [ ] } } 
{ "_id" : 4, "value" : { "userId" : 4, "ratings" : [ ] } } 
{ "_id" : 5, "value" : { "userId" : 5, "ratings" : [ ] } } 
{ "_id" : 6, "value" : { "userId" : 6, "ratings" : [ ] } } 
{ "_id" : 7, "value" : { "userId" : 7, "ratings" : [ ] } } 
{ "_id" : 8, "value" : { "userId" : 8, "ratings" : [ ] } } 
{ "_id" : 9, "value" : { "userId" : 9, "ratings" : [ ] } } 
{ "_id" : 10, "value" : { "userId" : 10, "ratings" : [ ] } } 
{ "_id" : 11, "value" : { "userId" : 11, "ratings" : [ ] } } 
{ "_id" : 12, "value" : { "userId" : 12, "ratings" : [ ] } } 
{ "_id" : 13, "value" : { "userId" : 13, "ratings" : [ ] } } 
{ "_id" : 14, "value" : { "userId" : 14, "ratings" : [ ] } } 
{ "_id" : 15, "value" : { "movieId" : 1, "movieRating" : 3 } } 
{ "_id" : 16, "value" : { "userId" : 16, "ratings" : [ ] } }

I am not getting the expected output. In fact, this output makes no sense to me!

Here is the python equivalent of what I am trying to do in the reducer (just in case the purpose of reducer wasn't clear above) :

def reducer_ratings_by_user(self, user_id, itemRatings):
        #Group (item, rating) pairs by userID
        ratings = []
        for movieID, rating in itemRatings:
            ratings.append((movieID, rating))
        yield user_id, ratings

Edit 1 @chridam

Here is an outline of what I really want to do here :

Movies.csv file looks like :

userId,movieId,movie-categoryId,reviewId,movieRating,reviewDate
1,1,1,1,5,7/12/2000
2,1,1,2,5,7/12/2000
3,1,1,3,5,7/12/2000
4,1,1,4,4,7/12/2000
5,1,1,5,4,7/12/2000
6,1,1,6,5,7/15/2000
1,2,1,7,4,7/25/2000
8,1,1,8,4,7/28/2000
9,1,1,9,3,8/3/2000
...
...

I import this into mongoDB :

mongoimport --db SomeName --collection ratings --type csv --headerline --file Movies.csv 

Then I am trying to apply the map-reduce function as define above. After that I will export it back to a csv by doing somethig like :

mongoexport --db SomeName --collection map_reduce_step1 --csv --out movie_ratings_out.csv --fields ...

This movie_ratings_out.csv file should be like :

userId, movieId1, rating1, movieId2, rating2 ,...
1,1,5,2,4
...
...

So each row contains all the (movie,rating) pair for every user.

Edit 2

Sample :

db.ratings.find().pretty()
{
    "_id" : ObjectId("57f4a0dd9cb74fc4d344a40f"),
    "userId" : 4,
    "movieId" : 1,
    "movie-categoryId" : 1,
    "reviewId" : 4,
    "movieRating" : 4,
    "reviewDate" : "7/12/2000"
}
{
    "_id" : ObjectId("57f4a0dd9cb74fc4d344a410"),
    "userId" : 5,
    "movieId" : 1,
    "movie-categoryId" : 1,
    "reviewId" : 5,
    "movieRating" : 4,
    "reviewDate" : "7/12/2000"
}
{
    "_id" : ObjectId("57f4a0dd9cb74fc4d344a411"),
    "userId" : 4,
    "movieId" : 2,
    "movie-categoryId" : 1,
    "reviewId" : 6,
    "movieRating" : 5,
    "reviewDate" : "7/15/2000"
}
{
    "_id" : ObjectId("57f4a0dd9cb74fc4d344a412"),
    "userId" : 4,
    "movieId" : 3,
    "movie-categoryId" : 1,
    "reviewId" : 2,
    "movieRating" : 5,
    "reviewDate" : "7/12/2000"
}
...

Then after MapReduce expected output json is :

{
    "_id" : ....,
    "userId" : 4,
    "movieList" : [ {
           "movieId" : 2
           "movieRating" : 5
         },
         {
           "movieId" : 1
           "movieRating" : 4
         }
         ...
        ]
   }
   {
    "_id" : ....,
    "userId" : 5,
    "movieList" : ...
   }
   ...