Update: Move field into another's field array - mongodb

Question

I'm not 100% percent if this is possible, but i think it should be.
I want to update a document so that a field (END) is inserted into an array (INTERMISSIONS). I think im quite close to achieving it, but i either get an error or insert the string "$END".
The two queries i have are :
db.myCollection.findOneAndUpdate(
{"_id" : new ObjectId("...")},
{ '$push' : {'INTERMISSIONS' : '$END' } }
)
This one completes without errors, but instead of the $END value just "$END" is inserted
db.myCollection.findOneAndUpdate(
{"_id" : new ObjectId("...")},
{ '$push' :
{'INTERMISSIONS' : {$first:"$END" }}
}
)
Here i tried to "force" mongo to recognise $END as a field, but i instead get the error The dollar ($) prefixed field '$first' in 'INTERMISSIONS..START.$first' is not valid for storage.
The document structure is the following
{
"_id" : ObjectId("5f7dabb9c02c0000d2003ec2"),
"USUARIO" : "admin",
"START" : ISODate("2020-10-07T11:51:21Z"),
"INTERMISSIONS" : [ ],
"END" : ISODate("2020-10-08T09:39:27Z")
}

Answer 1

When you use an operator check if it can take as argument an expression,
like here a field reference.
{ $push: { <field1>: <value1>, ... } }
MongoDB can do much more,with mongodb >= 4.2 you can do updates using the aggregation pipeline.
I give you the command,the "q" is the query,the "u" is the update pipeline,
you can use it with any driver that supports mongodb >= 4.2
> use testdb
switched to db testdb
> db.testcoll.drop()
true
> db.testcoll.insert(
... {
... "_id" : "5f7dabb9c02c0000d2003ec2",
... "USUARIO" : "admin",
... "START" :ISODate("2020-10-07T11:51:21Z"),
... "INTERMISSIONS" : [ ],
... "END" : ISODate("2020-10-08T09:39:27Z")
... }
... )
WriteResult({ "nInserted" : 1 })
> db.runCommand(
... {
... "update": "testcoll",
... "updates": [
... {
... "q": {},
... "u": [
... {
... "$addFields": {
... "INTERMISSIONS": {
... "$concatArrays": [
... "$INTERMISSIONS",
... [
... "$END" //or use {"start" : "$END"},anything you want to add
... ]
... ]
... }
... }
... }
... ],
... "multi": false
... }
... ]
... }
... )
{ "n" : 1, "nModified" : 1, "ok" : 1 }
> db.testcoll.find({}).pretty();
{
"_id" : "5f7dabb9c02c0000d2003ec2",
"USUARIO" : "admin",
"START" : ISODate("2020-10-07T11:51:21Z"),
"INTERMISSIONS" : [
ISODate("2020-10-08T09:39:27Z")
],
"END" : ISODate("2020-10-08T09:39:27Z")
}