CosmosDB $elemMatch syntax error - azure-cosmosdb

Question

I am getting a strange syntax error for some commands in the MongoDB API for CosmosDB. Say I have a collection called "Collection" with two documents:
{
"_id" : 1,
"arr" : [
{
"_id" : 11
},
{
"_id" : 12
}
]
}
{
"_id" : 2,
"arr" : [
{
"_id" : 21
},
{
"_id" : 22
}
]
}
If I try to run the query
db.getCollection('Collection').find( { _id : 2 }, { arr : { $elemMatch : { _id : 21 } } })
I get the result
{
"_t" : "OKMongoResponse",
"ok" : 0,
"code" : 9,
"errmsg" : "Syntax error, incorrect syntax near '10'.",
"$err" : "Syntax error, incorrect syntax near '10'."
}
But the command works perfectly fine on my locally hosted instance of MongoDB, returning the expected result:
{
"_id" : 2,
"arr" : [
{
"_id" : 21
}
]
}
Anyway, this is certainly not a syntax error, but there is no helpful error message. If this is not yet supported by CosmosDB, is there any way to only get certain embedded documents stored in an array?
If I try to use an aggregation pipeline to just extract the document in the array (I realize this should give a different result than the command above, but it would also work for my purposes), like so:
db.getCollection('Collection').aggregate([{ "$unwind" : "$arr" }, { "$match" : { "arr._id" : 21 } }] )
I get the result
{
"_t" : "OKMongoResponse",
"ok" : 0,
"code" : 118,
"errmsg" : "$match is currently only supported when it is the first and only stage of the aggregation pipeline. Please restructure your query to combine multiple $match stages into a single $match stage.",
"$err" : "$match is currently only supported when it is the first and only stage of the aggregation pipeline. Please restructure your query to combine multiple $match stages into a single $match stage."
}
So that doesn't work for me either.

Answer 1

Try this
db.collection.aggregate([
{
$match: {
"_id": 2
}
},
{
$project: {
arr: {
$filter: {
input: "$arr",
as: "ar",
cond: {
$eq: [
"$$ar._id",
21
]
}
}
}
}
}
])
Check it here