mongodb update document from first element of array - mongodb

Question

Consider a collection client with the following documents:
[
{
"id": 1,
"Name": "Susie",
"ownership" : {
"ownershipContextCode" : "C1"
},
"clientIds": [
{
"clientClusterCode": "clientClusterCode_1",
"clientId": "11"
}
]
},
{
"id": 2,
"Name": "John",
"ownership" : {
"ownershipContextCode" : "C2"
},
"clientIds": [
{
"clientClusterCode": "clientClusterCode_2",
"clientId": "22"
}
]
}
]
I am attempting to set a field (ownershipClientCode) as the first element of the clientIds array.
The result should be like that:
[
{
"id": 1,
"Name": "Susie",
"ownership" : {
"ownershipContextCode" : "C1",
"ownershipClientCode" : "clientClusterCode_1"
},
"clientIds": [
{
"clientClusterCode": "clientClusterCode_1",
"clientId": "11"
}
],
},
{
"id": 2,
"Name": "John",
"ownership" : {
"ownershipContextCode" : "C2",
"ownershipClientCode" : "clientClusterCode_2"
},
"clientIds": [
{
"clientClusterCode": "clientClusterCode_2",
"clientId": "22"
}
],
}
]
I'm using this query but I can't get sub object from the first element in the array
db.collection.aggregate([
{
$addFields: {
"Last Semester": {
"$arrayElemAt": [
"$clientIds",
0
]
}
}
}
])
This query add the all object but I want only the field (clientClusterCode).
Some thing like that
db.collection.aggregate([
{
$addFields: {
"Last Semester": {
"$arrayElemAt": [
"$clientIds",
0
].clientClusterCode
}
}
}
])
I'm using mongodb 4.0.0

Answer 1

You're very close: https://mongoplayground.net/p/HY1Pj0P4z12
db.collection.aggregate([
{
$addFields: {
"ownership.ownershipClientCode": {
"$arrayElemAt": [
"$clientIds.clientClusterCode",
0
]
}
}
}
])
You can use the dot notation within the $arrayElemAt as well as when you defining the field name.
To directly set the field, do something like this (use aggregation in the update): https://mongoplayground.net/p/js-usEJSH_A
db.collection.update({},
[
{
$set: {
"ownership.ownershipClientCode": {
"$arrayElemAt": [
"$clientIds.clientClusterCode",
0
]
}
}
}
],
{
multi: true
})
Note: The second method to update needs to be an array, so that it functions as an pipeline.