Aggregate distinct values in MongoDB - mongodb

Question

I have a mongodb db with 18625 collections. It has following keys:
"_id" : ObjectId("5aab14d2fc08b46adb79d99c"),
"game_id" : NumberInt(4),
"score_phrase" : "Great",
"title" : "NHL 13",
"url" : "/games/nhl-13/ps3-128181",
"platform" : "PlayStation 3",
"score" : 8.5,
"genre" : "Sports",
"editors_choice" : "N",
"release_year" : NumberInt(2012),
"release_month" : NumberInt(9),
"release_day" : NumberInt(11)
Now, i wish to create another dimension/ collection with only genres.
If i use the following query :
db.ign.aggregate([ {$project: {"genre":1}}, { $out: "dimen_genre" } ]);
It generates 18625 collections, even though there are only 113 distinct
genres.
How to apply distinct here and get the collection for genres with only the distinct 113 values.
I googled, bt it showed that aggregate and distinct don't work together in mongo.
I also tried : db.dimen_genre.distinct('genre').length
this showed that in dimension_genre, there are 113 distinct genres.
Precisely,
how to make a collection from existing one with only distinct values.
I am really new to NoSQLs.

Answer 1

You can use $addToSet to group unique values in one document and then $unwind to get back multiple docs:
db.ign.aggregate([
{
$group: {
_id: null,
genre: { $addToSet: "$genre" }
}
},
{
$unwind: "$genre"
},
{
$project: {
_id: 0
}
},
{ $out: "dimen_genre" }
]);

Answer 2

You can try
db.names.aggregate(
[
{ $group : { _id : "$genre", books: { $push: "$$ROOT" } } }
]
)
I have tried with Test and Sports as genre
It gives you output something like this
{
"_id" : "Test",
"books" : [
{
"_id" : ObjectId("5aaea6150cc1403ee9a02e0c"),
"game_id" : 4,
"score_phrase" : "Great",
"title" : "NHL 13",
"url" : "/games/nhl-13/ps3-128181",
"platform" : "PlayStation 3",
"score" : 8.5,
"genre" : "Test",
"editors_choice" : "N",
"release_year" : 2012,
"release_month" : 9,
"release_day" : 11
}
]
}
/* 2 */
{
"_id" : "Sports",
"books" : [
{
"_id" : ObjectId("5aaea3be0cc1403ee9a02d97"),
"game_id" : 4,
"score_phrase" : "Great",
"title" : "NHL 13",
"url" : "/games/nhl-13/ps3-128181",
"platform" : "PlayStation 3",
"score" : 8.5,
"genre" : "Sports",
"editors_choice" : "N",
"release_year" : 2012,
"release_month" : 9,
"release_day" : 11
},
{
"_id" : ObjectId("5aaea3c80cc1403ee9a02d9b"),
"game_id" : 4,
"score_phrase" : "Great",
"title" : "NHL 13",
"url" : "/games/nhl-13/ps3-128181",
"platform" : "PlayStation 3",
"score" : 8.5,
"genre" : "Sports",
"editors_choice" : "N",
"release_year" : 2012,
"release_month" : 9,
"release_day" : 11
},
{
"_id" : ObjectId("5aaea3cf0cc1403ee9a02d9f"),
"game_id" : 4,
"score_phrase" : "Great",
"title" : "NHL 13",
"url" : "/games/nhl-13/ps3-128181",
"platform" : "PlayStation 3",
"score" : 8.5,
"genre" : "Sports",
"editors_choice" : "N",
"release_year" : 2012,
"release_month" : 9,
"release_day" : 11
}
]
}