Any mongo experts here ?

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MongoDB

Hi all, I have a collection in a mongodb database which has the form

{
"_id" : ObjectId("57bffb4763e3de564a9f230d"),
        "id" : 29942,                                                                             
        "title" : "01 Ain't talkin about love",
        "genre" : "Classic Rock",
        "artist" : "NBPM",
        "album" : "New Band Practice Material",
        "duration" : 231.933
}

What I would like to produce is a listing grouped by artist and then album.

The following produces a list grouped by album

db.tracks.aggregate( [ { $group : { _id : "$album",tracks:{$push:'$title' } }} ] )

{
	"result" : [
		{
			"_id" : "The Hissing Of Summer Lawns",
			"tracks" : [
				"The Boho Dance",
				"Don't Interrupt The Sorrow",
				"Edith And The Kingpin",
				"Harry's House-Centerpiece",
				"The Hissing Of Summer Lawns",
				"In France They Kiss On Main Street",
				"The Jungle Line",
				"Shades Of Scarlett Conquering",
				"Shadows And Light",
				"Sweet Bird"
			]
		},
		{
			"_id" : "Shine",
			"tracks" : [
				"Bad Dreams",
				"Big Yellow Taxi (2007)",
				"Hana",
				"If",
				"If I Had A Heart",
				"Night Of The Iguana",
				"One Week Last Summer",
				"Shine",
				"Strong And Wrong",
				"This Place"
			]
		},
		{
			"_id" : "Hejira",
			"tracks" : [
				"Amelia",
				"A Strange Boy",
				"Black Crow",
				"Blue Motel Room",
				"Coyote",
				"Furry Sings the Blues",
				"Hejira",
				"Refuge of the Roads",
				"Song for Sharon"
			]
		},
		{
			"_id" : "Blue",
			"tracks" : [
				"A Case of You",
				"All I Want",
				"Blue",
				"California",
				"Carey",
				"The Last Time I Saw Richard",
				"Little Green",
				"My Old Man",
				"River",
				"This Flight Tonight"
			]
		}
	]
}

so I just need help with the final step to group by artist

What I have tried:

Everything shown in my code and googling