Puoi utilizzare l'aggregazione di seguito
db.users.aggregate([
{ "$match": { "_id": 2 }},
{ "$lookup": {
"from": "follows",
"let": { "id": "$_id" },
"pipeline": [
{ "$facet": {
"userFollows": [
{ "$match": { "$expr": { "$eq": ["$follower", "$$id"] }}}
],
"myFollows": [
{ "$match": { "$expr": { "$eq": ["$follower", 1] }}}
]
}},
{ "$project": {
"matchedFollowed": {
"$setIntersection": ["$userFollows.followed", "$myFollows.followed"]
}
}},
{ "$unwind": "$matchedFollowed" }
],
"as": "user"
}},
{ "$lookup": {
"from": "follows",
"let": { "ids": "$user.matchedFollowed" },
"pipeline": [
{ "$match": { "$expr": { "$in": ["$follower", "$$ids"] }}}
],
"as": "mutualConnections"
}}
])
