query collection A with filtered items from collection B - mongodb-query

Question

given the following 2 collections songs and play_log
i need a one mongo aggregation query with lookup to do the magic
to get the play log of all songs belonging to Bowie's "Scary Monsters" album with the song related info.
collection songs
[
{artist: 'David Bowie', title: 'Ashes to Ashes', album: 'Scary Monsters', year:'1980', track_number: 4 ,label: 'RCA Records'},
{artist: 'David Bowie', title: 'Fashion', album: 'Scary Monsters', year:'1980', track_number: 5 ,label: 'RCA Records'},
....
{artist: 'U2', title: 'Sunday Bloody Sunday', album: 'war', year '1983', track_number: 1, label: 'Island Records'},
{artist: 'U2', title: 'New Year's Day', album: 'war', year '1983', track_number: 3, label: 'Island Records'},
{artist: 'U2', title: 'The Refugee', album: 'war', year '1983', track_number: 6, label: 'Island Records'},
....
]
collection play_log
[
{ created: '2019-02-08T11:05:33', station: 'BBC Radio 6', artist: 'David Bowie', title: 'Ashes to Ashes' },
{ created: '2019-01-17T01:33:57', station: 'BBC Radio 1', artist: 'U2', title: 'Sunday Bloody Sunday' },
{ created: '2018-09-08T12:21:32', station: 'BBC Radio 2', artist: 'Morrissey', title: 'Every day is like Sunday' },
{ created: '2019-02-08T11:11:11', station: 'BBC Radio 4', artist: 'David Bowie', title: 'Fashion' },
...
]
expected result
[
{ created: '2019-02-08T11:05:33', station: 'BBC Radio 6', artist: 'David Bowie', title: 'Ashes to Ashes', album:'Scary Monsters', year:'1980', track_number: 4 ,label: 'RCA Records'},
{ created: '2019-02-08T11:11:11', station: 'BBC Radio 4', artist: 'David Bowie', title: 'Fashion', album: 'Scary Monsters', year:'1980', track_number: 5 ,label: 'RCA Records'},
...
]

Answer 1

This can be done through aggregation using $lookup operator as below:
db.play_log.aggregate([
// Join using artist fields
{
$lookup:
{
from: "songs",
localField: "artist",
foreignField: "artist",
as: "play_songs_logs"
}
},
// Filter any empty array found in newly created collection: play_songs_logs
{
$match: { "play_songs_logs": { $ne: [] } }
},
// Match only required album, this can be done before filter also if make aggregate on songs collection
{
$match:
{
"play_songs_logs.album" : "Scary Monsters"
}
},
// Push all elements or merged the elements
{
$replaceRoot: { newRoot: { $mergeObjects: [ { $arrayElemAt: [ "$play_songs_logs", 0 ] }, "$$ROOT" ] } }
},
// Filter not required fields
{ $project: { play_songs_logs: 0, _id:0 } }
])
Output of the above query:
{ "artist" : "David Bowie", "title" : "Ashes to Ashes", "album" : "Scary Monsters", "year" : "1980", "track_number" : 4, "label" : "RCA Records", "created" : "2019-02-08T11:05:33", "station" : "BBC Radio 6" }
{ "artist" : "David Bowie", "title" : "Fashion", "album" : "Scary Monsters", "year" : "1980", "track_number" : 4, "label" : "RCA Records", "created" : "2019-02-08T11:11:11", "station" : "BBC Radio 4" }
Before running above query, I have inserted the provided data and find query
> db.play_log.find()
{ "_id" : ObjectId("5c5f8c561765cd7b27eb4731"), "created" : "2019-02-08T11:05:33", "station" : "BBC Radio 6", "artist" : "David Bowie", "title" : "Ashes to Ashes" }
{ "_id" : ObjectId("5c5f8c561765cd7b27eb4732"), "created" : "2019-01-17T01:33:57", "station" : "BBC Radio 1", "artist" : "U2", "title" : "Sunday Bloody Sunday" }
{ "_id" : ObjectId("5c5f8c561765cd7b27eb4733"), "created" : "2018-09-08T12:21:32", "station" : "BBC Radio 2", "artist" : "Morrissey", "title" : "Every day is like Sunday" }
{ "_id" : ObjectId("5c5f8c561765cd7b27eb4734"), "created" : "2019-02-08T11:11:11", "station" : "BBC Radio 4", "artist" : "David Bowie", "title" : "Fashion" }
>
> db.songs.find()
{ "_id" : ObjectId("5c5f8c961765cd7b27eb4735"), "artist" : "David Bowie", "title" : "Ashes to Ashes", "album" : "Scary Monsters", "year" : "1980", "track_number" : 4, "label" : "RCA Records" }
{ "_id" : ObjectId("5c5f8c961765cd7b27eb4736"), "artist" : "David Bowie", "title" : "Fashion", "album" : "Scary Monsters", "year" : "1980", "track_number" : 5, "label" : "RCA Records" }
{ "_id" : ObjectId("5c5f8c961765cd7b27eb4737"), "artist" : "U2", "title" : "Sunday Bloody Sunday", "album" : "war", "year" : "1983", "track_number" : 1, "label" : "Island Records" }
{ "_id" : ObjectId("5c5f8c961765cd7b27eb4738"), "artist" : "U2", "title" : "New Year's Day", "album" : "war", "year" : "1983", "track_number" : 3, "label" : "Island Records" }
{ "_id" : ObjectId("5c5f8c961765cd7b27eb4739"), "artist" : "U2", "title" : "The Refugee", "album" : "war", "year" : "1983", "track_number" : 6, "label" : "Island Records" }
>
For details you can refer the official documents here: https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/