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// mondogb aggregation
[
{
'$lookup': {
'from': 'albums',
'localField': '_id',
'foreignField': 'singer_id',
'as': 'albums'
}
}, {
'$unwind': {
'path': '$albums',
'preserveNullAndEmptyArrays': false
}
}, {
'$lookup': {
'from': 'comments',
'localField': 'albums._id',
'foreignField': 'album_id',
'as': 'albums.comment'
}
}, {
'$group': {
'_id': '$_id',
'name': {
'$first': '$name'
},
'albums': {
'$push': '$albums'
}
}
}
]