Update: Move field into another's field array

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") 
}

Dharman · Accepted Answer · 2020-10-08 11:41:51Z

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")
}

edited Oct 8, 2020 at 11:41

Dharman♦

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answered Oct 8, 2020 at 11:35

Takis

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1 Comment

Juanjo Martinez Over a year ago

After tinkering with your query for a bit longer and trying to make it a "normal" query for the sake of standardization with the rest of my code, i assume there's no way to do this with an update query (other than using $out, which i guess it not specially efficient). If i'm mistaken please tell me so i don't have to look up everything again in a few months :)

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