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" } } })