Mongodb- Delete some of the array elements within embedded document - mongodb

Question

There are 2 kinds of documents.
Type 1. Documents contain the either MCN-ONE, MCN-TWO, MCN-THREE(or all 3) along with other values
2. Another type of documents do not contain any among these values.
First, I would like to get the documents having those array elements(either 1 or 2 or all 3). Then I want to keep MCN-ONE,MCN-TWO,MCN-THREE and delete all others (CCC-ALARM..etc) in bulk. Could you help to write the query? The below mentioned document falls in type 1.
{
"_id" : ObjectId("5d721f5296eaaafd1df263e8"),
"assetId" : "ALL",
"createdTime" : ISODate("2019-09-06T08:56:50.065Z"),
"default" : false,
"lastUpdatedTime" : ISODate("2019-09-06T09:11:35.463Z"),
"preferences" : {
"MCN-TWO" : [
"TEST"
],
"MCN-ONE" : [
"TEST",
"TEST",
"TEST"
],
"MCN-THREE" : [
"TEST"
],
"CCC-ALARM" : [
"TEST"
],
"SSD-ALARM" : [
"TEST"
],
"TFT-ALARM" : [
"TEST",
"TEST"
],
"REC-WARN" : []
}
}

Answer 1

The generic approach would be to transform preferences subdocument with $objectToArray, then filter desired elements with $filter, $map or $reduce and transform back with $arrayToObject. However, your requirement is "get elements MCN-ONE,MCN-TWO,MCN-THREE". The simple way is to update element preferences and replace just with conten of MCN-ONE,MCN-TWO,MCN-THREE. It can be done by this aggregation:
In order to filter documents, set the $match stage:
db.collection.aggregate(
[
{
$match: {
$expr: {
$or: [
{ $ne: ["$preferences.MCN-ONE", null] },
{ $ne: ["$preferences.MCN-TWO", null] },
{ $ne: ["$preferences.MCN-THREE", null] }
]
}
}
},
{
$set: {
preferences: {
$mergeObjects: [
{ "MCN-ONE": "$preferences.MCN-ONE" },
{ "MCN-TWO": "$preferences.MCN-TWO" },
{ "MCN-THREE": "$preferences.MCN-THREE" }
]
}
}
}
]
)