delete a specific object inside an array of object in mongodb, by id, not index. - not deleting

Question

I have a collection that has an array of objects - each with a UNIQUE ID - I want to be able to delete an item from the array - or a specific record. I have code that doesn't error - yet it does not remove the object from the array.

COLLECTION STRUCTURE

_id: 
,username: 
,prefs: [
    _id:
    ,title: 
]

DATA LOOKS LIKE

_id: 5a...46,
username: "bobsmith",
prefs: [
    {
        _id: 5...a,
        composition_title: "blah 1"
    }

    ,{
        _id: 5 c...2,
        composition_title: "blah 2"
    }

    ,{
        _id: 5 c...c,
        composition_title: "blah 3"
    }
]

CODE:

module.exports.removeUserPref = function (uid, pid, callback) {

    User.update(
        {
            '_id': mongoose.Types.ObjectId(uid)
        },
        {
            $pull: { pref: { $elemMatch: { _id: mongoose.Types.ObjectId(pid) } } }
        }
    , callback);

}

I know the User ID I am passing is correct and the Pref Id I am passing IS in the array - but My response is : "nModified": 0 and no deleted item

{
    "n": 1,
    "nModified": 0,
    "opTime": {
        "ts": "6641391849170796549",
        "t": 4
    },
    "electionId": "7fffffff0000000000000004",
    "ok": 1,
    "operationTime": "6641391849170796549",
    "$clusterTime": {
        "clusterTime": "6641391849170796549",
        "signature": {
            "hash": "sBXjaw1nw99+cfMIMVNq5KtCpt8=",
            "keyId": "6596828466803376130"
        }
    }
}

Remove $elemMatch. It should be { $pull: { pref: { _id: mongoose.Types.ObjectId(pid) } } } — Ashh
– Ashh, Commented Jan 1, 2019 at 5:35

Ashh · Accepted Answer · 2019-01-01 05:40:30Z

1

$elemMatch is an query and projection operator. There is nothing to do it with the update operators.

So instead try to do something like this

User.update(
  { "_id": mongoose.Types.ObjectId(uid) },
  { "$pull": { "pref": { "_id": mongoose.Types.ObjectId(pid) } } }
)

answered Jan 1, 2019 at 5:40

Ashh

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