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db.collection.aggregate([
{ "$lookup": {
"from": Albums.collection.name,
"let": { "albums": "$albums" },
"pipeline": [
{ "$match": { "$expr": { "$in": [ "$_id", "$$albums" ] } } },
{ "$lookup": {
"from": Songs.collection.name,
"let": { "songs": "$songs" },
"pipeline": [
{ "$match": { "$expr": { "$in": [ "$_id", "$$songs" ] } } }
],
"as": "songs"
}}
],
"as": "albums"
}}
])