How do I remove an array item in Mongodb / Golang?

I have the following data structure, and I'm attempting to remove an item from the 'artists' array.

[
    {
        "id": "56b26eeb4a876400011369e9",
        "name": "Ewan Valentine",
        "email": "[email protected]",
        "artists": [
            "56b26f334a876400011369ea",
            "56b2702881318d0001dd1441",
            "56b2746fdf1d7e0001faaa92",
        ],
        "user_location": "Manchester, UK"
    }
]

Here's my function...

// Remove artist from user
func (repo *UserRepo) RemoveArtist(userId string, artistId string) error {
    change := bson.M{"artists": bson.M{"$pull": bson.ObjectIdHex(artistId)}}
    fmt.Println(userId)
    err := repo.collection.UpdateId(bson.ObjectIdHex(userId), change)
    return err
}

I'm getting the following error:

{
  "_message": {
    "Err": "The dollar ($) prefixed field '$pull' in 'artists.$pull' is not valid for storage.",
    "Code": 52,
    "N": 0,
    "Waited": 0,
    "FSyncFiles": 0,
    "WTimeout": false,
    "UpdatedExisting": false,
    "UpsertedId": null
  }
}

Solution

The $pull operator is a "top level" operator in update statements, so you simply have this the wrong way around:

    change := bson.M{"$pull": bson.M{"artists": bson.ObjectIdHex(artistId)}}

The order of update operators is always operator first, action second.

If there is no operator at the "top level" keys, MongoDB interprets this as just a "plain object" to update and "replace" the matched document. Hence the error about the $ in the key name.