Filter Array Content to a Query containing $concatArrays - mongodb

Question

Given this function, I have a data set that I am querying. The data looks like this:
db.activity.insert(
{
"_id" : ObjectId("5908e64e3b03ca372dc945d5"),
"startDate" : ISODate("2017-05-06T00:00:00Z"),
"details" : [
{
"code" : "2",
"_id" : ObjectId("5908ebf96ae5003a4471c9b2"),
"walkDistance" : "03",
"jogDistance" : "01",
"runDistance" : "08",
"sprintDistance" : "01"
}
]
}
)
db.activity.insert(
{
"_id" : ObjectId("58f79163bebac50d5b2ae760"),
"startDate" : ISODate("2017-05-07T00:00:00Z"),
"details" : [
{
"code" : "2",
"_id" : ObjectId("58f7948fbebac50d5b2ae7f2"),
"walkDistance" : "01",
"jogDistance" : "02",
"runDistance" : "09",
"sprintDistance" : ""
}
]
}
)
Using this function, thanks to Neil Lunn, I am able to get my desired output:
db.activity.aggregate([
{ "$project": {
"_id": 0,
"unique": {
"$filter": {
"input": {
"$setDifference": [
{ "$concatArrays": [
"$details.walkDistance",
"$details.jogDistance",
"$details.runDistance",
"$details.sprintDistance"
]},
[]
]
},
"cond": { "$ne": [ "$$this", "" ] }
}
}
}},
{ "$unwind": "$unique" },
{ "$group": {
"_id": null,
"uniqueArray": { "$addToSet": "$unique" }
}}
])
However, I cannot add a match statement to the beginning.
db.activity.aggregate([
{$match: {"startDate" : ISODate("2017-05-06T00:00:00Z"), "details.code": "2" },
{$unwind: '$details'},
{$match: {"startDate" : ISODate("2017-05-06T00:00:00Z"), "details.code": "2" },
{ "$project": {
"_id": 0,
"unique": {
"$filter": {
"input": {
"$setDifference": [
{ "$concatArrays": [
"$details.walkDistance",
"$details.jogDistance",
"$details.runDistance",
"$details.sprintDistance"
]},
[]
]
},
"cond": { "$ne": [ "$$this", "" ] }
}
}
}},
{ "$unwind": "$unique" },
{ "$group": {
"_id": null,
"uniqueArray": { "$addToSet": "$unique" }
}}
])
Because it gives an error message of:
> $concatArrays only supports arrays, not string
How can I modify this query so that a $match statement can be added?

Answer 1

Don't $unwind the array you are feeding to $concatArrays. Instead apply $filter to only extract the matching values. And as stated, we can just use $setUnion for the 'unique concatenation' instead:
db.activity.aggregate([
{ "$match": { "startDate" : ISODate("2017-05-06T00:00:00Z"), "details.code": "2" } },
{ "$project": {
"_id": 0,
"unique": {
"$let": {
"vars": {
"filtered": {
"$filter": {
"input": "$details",
"cond": { "$eq": [ "$$this.code", "2" ] }
}
}
},
"in": {
"$setDifference": [
{ "$setUnion": [
"$$filtered.walkDistance",
"$$filtered.jogDistance",
"$$filtered.runDistance",
"$$filtered.sprintDistance"
]},
[""]
]
}
}
}
}},
{ "$unwind": "$unique" },
{ "$group": {
"_id": null,
"uniqueArray": { "$addToSet": "$unique" }
}}
])
Using $let makes things a bit cleaner syntax wise since you don't need to specify multiple $map and $filter statements "inline" as the source for $setUnion