MapReduce: aggregate in map function? - mongodb

Question

Suppose you have a DB where every document is a tweet from Twitter, and you want, with MapReduce, to generate another document that contains:
Number of tweets published on every country
List of words contained in those tweets, with a counter that counts the total hits of that word. This, for every country too.
My question: is it fine to aggregate and count the words on the map function, and then again on the reduce function? Doing it like this, the output of the map function represents the information of a single tweet, and the reduce function aggregates the info from several tweets, all from the same country, but I don't know if this is a good practice with the MapReduce algorithm...
Thank you in advance!

Answer 1

In mongoDB 3.4 you can do this process with aggregation framework.
For the first bullet, you just have to use $group operator at the country field and count the tweets.
For the second bullet, you have to use $split(new in 3.4) operator at the field of the tweet text, then use $unwind at the generated array and finally use $group with word as _id or country + word as _id.
If you have an older version of mongodb then you have to use map-reduce procedure but, have in mind, aggregation framework is much faster than map-reduce at mongodb.
$split: https://docs.mongodb.com/manual/reference/operator/aggregation/split/#exp._S_split
$unwind: https://docs.mongodb.com/manual/reference/operator/aggregation/unwind/
$group: https://docs.mongodb.com/manual/reference/operator/aggregation/group/

Answer 2

Building from the great answer above by Moi Syme, you ideally would run the following aggregate operation to get the desired result:
db.tweets.aggregate([
{ "$project": { "wordList": { "$split": [ "$text", " " ] }, "user.country": 1 } },
{ "$unwind": "$wordList" },
{
"$group": {
"_id": {
"country": "$user.country",
"word": "$wordList"
},
"count": { "$sum": 1 }
}
},
{
"$group": {
"_id": "$_id.country",
"numberOfTweets": { "$sum": 1 },
"counts": {
"$push": {
"word": "$_id.word",
"count": "$count"
}
}
}
}
])