Merge documents from 2 collections in MongoDB & preserve property of a field - mongodb

Question

I have two collections, 1. temporaryCollection, 2. permanentCollection, I would like to take data from temporaryCollection and update in permanentCollection. To see the expected result see updatedPermanentCollection below.
Fields that are taken from Temporary collection and updated in Permanent collection are:
emailAddresses
phoneNumbers
ContactName
ContactNumber
For your info, the fields that are changed in Temporary collection
contacts[0]['emailAddresses']
contacts[0]['ContactName']
contacts[0]["phoneNumbers"]
contacts[0]["ContactNumber"]
Field that are that should not be changed after updation in UpdatedPermanentCollection is
contacts._id
Note: contacts is an Array of objects, for simplicity I have shown just one object.
I am currently using the below query which updates the permanentCollection but also overrides the contacts._id field. I don't want the contacts._id field to be overridden.
Here is my MongoDB Query
db.temporaryCollection.aggregate([
{
$match: {
userID: ObjectId("61d1efea2c0fab00340f47c8"),
},
},
{
$merge: {
into: "permanentCollection",
on: "userID",
whenMatched: "merge",
whenNotMatched: "insert",
},
},
]);
1. temporaryCollection
{
"_id": { "$oid": "61d1f04266289f003452d705" },
"userID": { "$oid": "61d1efea2c0fab00340f47c8" },
"contacts": [
{
"emailAddresses": [
{ "id": "6884", "label": "email1", "email": "addedemail#gmail.com" }
],
"phoneNumbers": [
{
"label": "other",
"id": "4594",
"number": "[phone]"
},
{
"label": "other",
"id": "4595",
"number": "[phone]"
}
],
"_id": { "$oid": "61d1f04266289f003452d744" },
"ContactName": "Sample User 1 Name Changed",
"ContactNumber": "[phone]",
"recordID": "833"
}
],
"userNumber": "[phone]",
"__v": 7
}
2. permanentCollection
{
"_id": { "$oid": "61d1f04266289f003452d701" },
"userID": { "$oid": "61d1efea2c0fab00340f47c8" },
"contacts": [
{
"emailAddresses": [],
"phoneNumbers": [
{
"label": "other",
"id": "4594",
"number": "[phone]"
},
{
"label": "other",
"id": "4595",
"number": "[phone]"
}
],
"_id": { "$oid": "61d1f04266289f003452d722" },
"ContactName": "Sample User 1",
"ContactNumber": "[phone]",
"recordID": "833"
}
],
"userNumber": "[phone]",
"__v": 7
}
3. updatedPermanentCollection (Expected result)
{
"_id": { "$oid": "61d1f04266289f003452d701" },
"userID": { "$oid": "61d1efea2c0fab00340f47c8" },
"contacts": [
{
"emailAddresses": [
{ "id": "6884", "label": "email1", "email": "addedemail#gmail.com" }
],
"phoneNumbers": [
{
"label": "other",
"id": "4594",
"number": "[phone]"
},
{
"label": "other",
"id": "4595",
"number": "[phone]"
}
],
"_id": { "$oid": "61d1f04266289f003452d722" },
"ContactName": "Sample User 1 Name Changed",
"ContactNumber": "[phone]",
"recordID": "833"
}
],
"userNumber": "[phone]",
"__v": 7
}

Answer 1

Try with this aggregation query.
db.temporarCollection.aggreagate(
[
{
"$lookup": {
"from": "permanantCollection",
"let": {
"user_id": "$userID"
},
"pipeline": [
{
"$match": {
"$expr": {
"$eq": [
"$$user_id", "$userID"
]
}
}
}
],
"as": "pcontacts"
}
}, {
"$unwind": {
"path": "$pcontacts",
"preserveNullAndEmptyArrays": true
}
}, {
"$project": {
"contacts": {
"$map": {
"input": "$contacts",
"as": "contact",
"in": {
"tcontact": "$$contact",
"pcontact": {
"$first": {
"$filter": {
"input": "$pcontacts.contacts",
"as": "pcontact",
"cond": {
"$eq": [
"$$pcontact.recordID", "$$contact.recordID"
]
}
}
}
}
}
}
},
"userNumber": 1,
"userID": 1,
"_id": 0
}
}, {
"$project": {
"contacts": {
"$map": {
"input": "$contacts",
"as": "contact",
"in": {
"emailAddresses": "$$contact.tcontact.emailAddresses",
"phoneNumbers": "$$contact.tcontact.phoneNumbers",
"ContactName": "$$contact.tcontact.ContactName",
"ContactNumber": "$$contact.tcontact.ContactNumber",
"recordID": {
"$let": {
"vars": {},
"in": {
"$cond": {
"if": "$$contact.pcontact.recordID",
"then": "$$contact.pcontact.recordID",
"else": "$$contact.tcontact.recordID"
}
}
}
},
"_id": {
"$let": {
"vars": {},
"in": {
"$cond": {
"if": "$$contact.pcontact._id",
"then": "$$contact.pcontact._id",
"else": "$$contact.tcontact._id"
}
}
}
}
}
}
},
"userNumber": 1,
"userID": 1
}
}, {
"$merge": {
"into": "pc",
"on": "userID",
"whenMatched": "replace",
"whenNotMatched": "insert"
}
}
])
It is not a fully optimized query but it works.

Answer 2

Try to add $unset to db query.
db.temporaryCollection.aggregate([
{
$unset: "_id"
},
{
$match: {
userID: ObjectId("61d1efea2c0fab00340f47c8"),
},
},
{
$merge: {
into: "permanentCollection",
on: "userID",
whenMatched: "merge",
whenNotMatched: "insert",
},
},
]);