PyMongo(3.6.1) - db.collection.aggregate(pipeline) NOT WORKING - mongodb

Question

I'm having trouble with PyMongo, I have been googling for hours, but found no solution.
I'm working with some Python Scripts just to practice with MongoDb, which is running on my local machine. I have populated my mongoDb instance with one database "moviesDB", which contains 3 different collections:
1.Movies collection, here is an example of a document from this coll:
{'_id': 1,
'title': 'Toy Story (1995)',
'genres': ['Adventure', 'Animation', 'Children', 'Comedy', 'Fantasy'],
'averageRating': 3.87,
'numberOfRatings': 247,
'tags': [
{'_id': ObjectId('5b04970b87977f1fec0fb6e9'),
'userId': 501,
'tag': 'Pixar',
'timestamp': 1292956344}
]
}
2.Ratings collection, which looks like this:
{ '_id':ObjectId('5b04970c87977f1fec0fb923'),
'userId': 1,
'movieId': 31,
'rating': 2.5,
'timestamp': 1260759144}
3.Tags collection, that I won't use here, so it's not important.
Now, what I'm trying to do is: given a user (in this example, user 1), find all the genres of movies that he rated and per each genre list all the movieIds regarding that genre.
Here's my code:
"""
This query basically retrieves movieIds,
so from the result list of several documents like this:
{
ObjectId('5b04970c87977f1fec0fb923'),
'userId': 1,
'movieId': 31,
'rating': 2.5,
'timestamp': 1260759144},
retrieves only an array of integers, where each number represent a movie
that the user 1 rated."""
movies_rated_by_user = list(db.ratings.distinct(movieId, {userId: 1}))
pipeline = [
{"$match": {"_id ": {"$in": movies_rated_by_user}}},
{"$unwind": "$genres"},
{"$group": {"_id": "$genres", "movies": {"$addToSet": "$_id"}}}]
try:
"""HERE IS THE PROBLEM, SINCE db.movies.aggregate() RETURNS NOTHING!
so the cursor is empty."""
cursor = db.movies.aggregate(pipeline, cursor={})
except OperationFailure:
print("Something went Wrong", file=open("operations_log.txt", "a"))
print(OperationFailure.details, file=open("operations_log.txt", "a"))
sys.exit(1)
aggregate_genre = []
for c in cursor:
aggregate_genre.append(c)
print(aggregate_genre)
The point is that the aggregate function on the movies collection retrieves NOTHING, whereas it really should, since I tried this query on the MongoShell and it worked just fine. Here's how the mongoDB shell-query looks like:
db.movies.aggregate(
[
{$match:{_id : {$in: ids}}},
{$unwind : "$genres"},
{$group :
{
_id : "$genres",
movies: { $addToSet : "$_id" }}}
]
);
Where the 'ids' variables is defined like this, just like the movies_rated_by_user variable in the code:
ids= db.ratings.distinct("movieId", {userId : 1});
The result from the aggregate method looks like this (this is what the aggregate_genre variable in the code, should contain):
{ "_id" : "Western", "movies" : [ 3671 ] }
{ "_id" : "Crime", "movies" : [ 1953, 1405 ] }
{ "_id" : "Fantasy", "movies" : [ 2968, 2294, 2193, 1339 ] }
{ "_id" : "Comedy", "movies" : [ 3671, 2294, 2968, 2150, 1405 ] }
{ "_id" : "Sci-Fi", "movies" : [ 2455, 2968, 1129, 1371, 2105 ] }
{ "_id" : "Adventure", "movies" : [ 2193, 2150, 1405, 1287, 2105, 2294,
2968, 1371, 1129 ] }
Now the problem is the aggregate method, is there any error with the pipeline string??
PLEASE HELP!!
Thank you

Answer 1

I think you need to cast the cursor to a list before iterating through it. Hope this helps.
cursor = list(db.movies.aggregate(pipeline, cursor={}))