Extract a value from an object in array matching a condition - mongodb

Question

I need help to make an aggregation pipeline in mongodb.
The mongodb version i'm using is 4.
The documents stored in database looks like this:
[{
_id : "xxxxxx",
names : [
{ "lang" : "EN", value : "foo" },
{ "lang" : "IT", value : "bar" },
{ "lang" : "NOLANG", value : "baz" }
],
some : "value"
},{
_id : "yyyyyy",
names : [
{ "lang" : "FR", value : "quux" },
{ "lang" : "IT", value : "quuux" },
{ "lang" : "NOLANG", value : "quuuux" }
],
some : "value"
}]
I need to add a field with aggregation that contains the value of a certain language (for this example i'll take "EN"), if no element with requested language is found i need to get the "NOLANG" object value.
So, the result of the aggregation must looks like:
[{
_id : "xxxxxx",
name : "foo",
some : "value"
},{
_id : "yyyyyy",
name : "quuuux",
some : "value"
}]
This is the pipeline i wrote:
[
{
$project : {
names : 0,
name: {
$filter: {
input: '$names',
as: 'name',
cond: {
$switch: {
$branches: [
{
case : {
$eq : ["$$name.lang", "EN"]
},
then : "$$name.value"
} ,{
case : {
$eq : ["$$name.lang", "NOLANG"]
},
then : "$$name.value"
}
],
default : ''
}
}
}
}
}
}
]
It gives me the error: Expected "[" or AggregationStage but "{" found.
What i'm doing wrong? Someone can help me please?
Thanks

Answer 1

You can use below aggregation
db.collection.aggregate([
{ "$project": {
"some": 1,
"name": {
"$arrayElemAt": [
"$names.value",
{
"$cond": [
{
"$ne": [
{ "$indexOfArray": ["$names.lang", "EN"] },
-1
]
},
{ "$indexOfArray": ["$names.lang", "EN"] },
{ "$indexOfArray": ["$names.lang", "NOLANG"] }
]
}
]
}
}}
])
Output
[
{
"_id": "xxxxxx",
"name": "baz",
"some": "value"
},
{
"_id": "yyyyyy",
"name": "quuuux",
"some": "value"
}
]