mongodb $aggregate empty array and multiple documents - mongodb

Question

mongodb has below document:
> db.test.find({name:{$in:["abc","abc2"]}})
{ "_id" : 1, "name" : "abc", "scores" : [ ] }
{ "_id" : 2, "name" : "abc2", "scores" : [ 10, 20 ] }
I want get scores array length for each document, how should I do?
Tried below command:
db.test.aggregate({$match:{name:"abc2"}}, {$unwind: "$scores"}, {$group: {_id:null, count:{$sum:1}}} )
Result:
{ "_id" : null, "count" : 2 }
But below command:
db.test.aggregate({$match:{name:"abc"}}, {$unwind: "$scores"}, {$group: {_id:null, count:{$sum:1}}} )
Return Nothing. Question:
How should I get each lenght of scores in 2 or more document in one
command?
Why the result of second command return nothing? and how
should I check if the array is empty?

Answer 1

So this is actually a common problem. The result of the $unwind phase in an aggregation pipeline where the array is "empty" is to "remove" to document from the pipeline results.
In order to return a count of "0" for such an an "empty" array then you need to do something like the following.
In MongoDB 2.6 or greater, just use $size:
db.test.aggregate([
{ "$match": { "name": "abc" } },
{ "$group": {
"_id": null,
"count": { "$sum": { "$size": "$scores" } }
}}
])
In earlier versions you need to do this:
db.test.aggregate([
{ "$match": { "name": "abc" } },
{ "$project": {
"name": 1,
"scores": {
"$cond": [
{ "$eq": [ "$scores", [] ] },
{ "$const": [false] },
"$scores"
]
}
}},
{ "$unwind": "$scores" },
{ "$group": {
"_id": null,
"count": { "$sum": {
"$cond": [
"$scores",
1,
0
]
}}
}}
])
The modern operation is simple since $size will just "measure" the array. In the latter case you need to "replace" the array with a single false value when it is empty to avoid $unwind "destroying" this for an "empty" statement.
So replacing with false allows the $cond "trinary" to choose whether to add 1 or 0 to the $sum of the overall statement.
That is how you get the length of "empty arrays".

Answer 2

To get the length of scores in 2 or more documents you just need to change the _id value in the $group pipeline which contains the distinct group by key, so in this case you need to group by the document _id.
Your second aggregation returns nothing because the $match query pipeline passed a document which had an empty scores array. To check if the array is empty, your match query should be
{'scores.0': {$exists: true}} or {scores: {$not: {$size: 0}}}
Overall, your aggregation should look like this:
db.test.aggregate([
{ "$match": {"scores.0": { "$exists": true } } },
{ "$unwind": "$scores" },
{
"$group": {
"_id": "$_id",
"count": { "$sum": 1 }
}
}
])