Mongodb $push in nested array

Probably something like this where ID is your ObjectId. The first {} are necessary to identify your document. It is not required to use an ObjectId as long as you have another unique identifier in your collection.

db.collection.update(
    { "_id": ID, "playlists._id": "58"},
    { "$push": 
        {"playlists.$.musics": 
            {
                "name": "test name",
                "duration": "4.00"
            }
        }
    }
)

This way it worked for me!

"playlists.$[].musics":

db.collection.update(
{ "_id": ID, "playlists._id": "58"},
{ "$push": 
    {"playlists.$[].musics": 
        {
            "name": "test name",
            "duration": "4.00"
        }
    }
 }
)

https://docs.mongodb.com/manual/reference/operator/update/positional-filtered/#position-nested-arrays-filtered