Mongodb aggregate $group for non-existing items - mongodb

Question

I have document like this :
Documents :
{score: 1, value: 10}
{score: 3, value: 10}
{score: 1, value: 10}
{score: 4, value: 10}
{score: 1, value: 10}
{score: 5, value: 10}
{score: 5, value: 10}
{score: 10, value: 10}
In this collection, there is no score for 2,6,7,8,9 but I need output like below.
Output :
{score: 1, avg: 10}
{score: 2, avg: 0}
{score: 3, avg: 10}
{score: 4, avg: 10}
{score: 5, avg: 10}
{score: 6, avg: 0}
{score: 7, avg: 0}
{score: 8, avg: 0}
{score: 9, avg: 0}
{score: 10, avg: 10}
Any option in Mongo aggregate which will generate this. Please assist

Answer 1

You can try that using aggregation :
db.collection.aggregate([
{ $group: { _id: '$score', avg: { $avg: '$value' } } },
{ $group: { _id: '', min: { $min: '$_id' }, max: { $max: '$_id' }, data: { $push: '$$ROOT' } } },
{ $project: { _id: 0, data: 1, nums: { $range: ['$min', "$max", 1] } } },
{ $project: { data: { $concatArrays: ["$data", { $map: { input: { $setDifference: ["$nums", "$data._id"] }, in: { _id: '$$this', avg: 0 } } }] } } },
{ $unwind: '$data' }, { $replaceRoot: { newRoot: "$data" } }
])
Test : MongoDB-Playground

Answer 2

Assuming you know the range of scores, there's a trick to achieve exactly what you want :
1 - Insert in your collection a document for each score, with value field not set or set to null :
db.collection.insertMany([
{
score: 1,
},
{
score: 2,
},
{
score: 3,
},
{
score: 4,
},
{
score: 5,
},
{
score: 6,
},
{
score: 7,
},
{
score: 8,
},
{
score: 9,
},
{
score: 10,
}
]);
It's important for value field not to be set, because a value set at 0 will affect average calculation
Of course this operation must be performed only once.
Then you can apply the following aggregation, which will output exactly what you need :
db.collection.aggregate([
{
$bucket: {
groupBy: "$score",
boundaries: [
0,
1,
2,
3,
4,
5,
6,
7,
8,
9,
10,
11
],
output: {
avg: {
$avg: "$value"
}
}
}
},
{
$project: {
score: "$_id",
avg: {
$ifNull: [
"$avg",
0
]
},
_id: 0
}
}
])
Will output :
[
{
"avg": 10,
"score": 1
},
{
"avg": 0,
"score": 2
},
{
"avg": 10,
"score": 3
},
{
"avg": 10,
"score": 4
},
{
"avg": 10,
"score": 5
},
{
"avg": 0,
"score": 6
},
{
"avg": 0,
"score": 7
},
{
"avg": 0,
"score": 8
},
{
"avg": 0,
"score": 9
},
{
"avg": 10,
"score": 10
}
]
You can test it here.