MongoDB creates array of arrays in $group $push instead of flat array - mongodb

Question

I am trying to group a set of documents after an $unwind operation. My documents look like this:
{
"_id" : ObjectId("5cdb5b5acadf5100019da2f4"),
"allowedLocations" : [
{
"type" : "country",
"value" : "world",
"label" : "World"
}
],
"disallowedLocations" : [
{
"type" : "country",
"value" : "CF",
"label" : "Central African Republic"
},
{
"type" : "country",
"value" : "CN",
"label" : "China"
}
],
}
{
"_id" : ObjectId("5cdb5b5acadf5100019da2f4"),
"allowedLocations" : [
{
"type" : "country",
"value" : "US",
"label" : "United States of America"
}
],
"disallowedLocations" : [
{
"type" : "country",
"value" : "CA",
"label" : "Canada"
},
{
"type" : "country",
"value" : "MX",
"label" : "Mexico"
}
],
}
I want to group them by _id and then concatenate the allowedLocations and disallowedLocations arrays into one. The group stage in my pipeline looks like this:
{
"$group" : {
"_id" : "$_id",
"allowedLocations" : {
"$push" : "$allowedLocations"
},
"disallowedLocations" : {
"$push" : "disallowedLocations"
}
}
}
The problem is, the result I get is not a document with both arrays concatenated, but an array of arrays, each element of the array being the array of each document:
{
"_id" : ObjectId("5cdb5b5acadf5100019da2f4"),
"allowedLocations" : [
[
{
"type" : "country",
"value" : "US",
"label" : "United States of America"
}
],
[
{
"type" : "country",
"value" : "world",
"label" : "World"
}
],
],
"disallowedLocations" : [
[
{
"type" : "country",
"value" : "CF",
"label" : "Central African Republic"
},
{
"type" : "country",
"value" : "CN",
"label" : "China"
}
],
[
{
"type" : "country",
"value" : "CA",
"label" : "Canada"
},
{
"type" : "country",
"value" : "MX",
"label" : "Mexico"
}
]
}
}
Is there a way to produce a flat array with only objects as elements? I also tried with $concatArrays before the push but that creates more arrays inside the arrays.

Answer 1

Two solutions here. You can either run $unwind on both arrays to get single allowed and disallowed location per document and then run your $group stage:
db.col.aggregate([
{
$unwind: "$allowedLocations"
},
{
$unwind: "$disallowedLocations"
},
{
"$group" : {
"_id" : "$_id",
"allowedLocations" : {
"$addToSet" : "$allowedLocations"
},
"disallowedLocations" : {
"$addToSet" : "$disallowedLocations"
}
}
}
])
or you can run your $group first and then use $reduce to flatten allowedLocations and disallowedLocations:
db.col.aggregate([
{
"$group" : {
"_id" : "$_id",
"allowedLocations" : {
"$push" : "$allowedLocations"
},
"disallowedLocations" : {
"$push" : "$disallowedLocations"
}
}
},
{
$project: {
_id: 1,
allowedLocations: {
$reduce: {
input: "$allowedLocations",
initialValue: [],
in: { $concatArrays: [ "$$value", "$$this" ] }
}
},
disallowedLocations: {
$reduce: {
input: "$disallowedLocations",
initialValue: [],
in: { $concatArrays: [ "$$value", "$$this" ] }
}
}
}
}
])