How to group documents on index of array elements? - mongodb

Question

I'm looking for a way to take data such as this
{ "_id" : 5, "count" : 1, "arr" : [ "aga", "dd", "a" ] },
{ "_id" : 6, "count" : 4, "arr" : [ "aga", "ysdf" ] },
{ "_id" : 7, "count" : 4, "arr" : [ "sad", "aga" ] }
I would like to sum the count based on the 1st item(index) of arr. In another aggregation I would like to do the same with the 1st and the 2nd item in the arr array.
I've tried using unwind, but that breaks up the data and the hierarchy is then lost.
I've also tried using
$group: {
_id: {
arr_0:'$arr.0'
},
total:{
$sum: '$count'
}
}
but the result is blank arrays

Answer 1

Actually you can't use the dot notation to group your documents by element at a specified index. To two that you have two options:
First the optimal way using the $arrayElemAt operator new in MongoDB 3.2. which return the element at a specified index in the array.
db.collection.aggregate([
{ "$group": {
"_id": { "$arrayElemAt": [ "$arr", 0 ] },
"count": { "$sum": 1 }
}}
])
From MongoDB version 3.0 backward you will need to de-normalise your array then in the first time $group by _id and use the $first operator to return the first item in the array. From there you will need to regroup your document using that value and use the $sum to get the sum. But this will only work for the first and last index because MongoDB also provides the $last operator.
db.collection.aggregate([
{ "$unwind": "$arr" },
{ "$group": {
"_id": "$_id",
"arr": { "$first": "$arr" }
}},
{ "$group": {
"_id": "$arr",
"count": { "$sum": 1 }
}}
])
which yields something like this:
{ "_id" : "sad", "count" : 1 }
{ "_id" : "aga", "count" : 2 }
To group using element at position p in your array you will get a better chance using the mapReduce function.
var mapFunction = function(){ emit(this.arr[0], 1); };
var reduceFunction = function(key, value) { return Array.sum(value); };
db.collection.mapReduce(mapFunction, reduceFunction, { "out": { "inline": 1 } } )
Which returns:
{
"results" : [
{
"_id" : "aga",
"value" : 2
},
{
"_id" : "sad",
"value" : 1
}
],
"timeMillis" : 27,
"counts" : {
"input" : 3,
"emit" : 3,
"reduce" : 1,
"output" : 2
},
"ok" : 1
}