Renaming Field Within an Array in MongoDB - mongodb

Question

I need to update the name of field in a collection. The problem is that the field in question is within an array. So I'm trying to determine the correct way do this. I tried this to accomplish renaming the field that exists within the "plans" array: :
db.customers.updateMany( {}, { $rename: { "plans" : { "subscriptionType": "membershipType" } } } );
But this won't work. What's the correct way to handle this kind of transformation of a field within an array?
The data looks like this:
{
_id: 123,
prop1: value,
prop2: value,
prop3: value,
plans: [
subscriptionType: value,
otherProp: value,
otherProp: value
]
}

Answer 1

You can use Aggregation Framework's $addFields to override plans field and $map operator to rename field inside an array. Then you can use $out to override existing collection:
db.customers.aggregate([
{
$addFields: {
plans: {
$map:{
input: "$plans",
as: "plan",
in: {
membershipType: "$$plan.subscriptionType",
otherField: "$$plan.otherField",
otherField2: "$$plan.otherField2"
}
}
}
}
},
{
$out: "customers"
}
])
Alternatively you can do that dynamically. In this solution you don't have to explicitly specify other field names:
db.customers.aggregate([
{
$addFields: {
plans: {
$map:{
input: "$plans",
as: "plan",
in: {
$mergeObjects: [
{ membershipType: "$$plan.subscriptionType" },
{
$arrayToObject: {
$filter: {
input: { $objectToArray: "$$plan" },
as: "plan",
cond: { $ne: [ "$$plan.k", "subscriptionType" ] }
}
}
}
]
}
}
}
}
},
{
$out: "customers"
}
])
Using $objectToArray to $filter out old key-value pair and the using $mergeObjects to combine that filtered object with new renamed field.