db={"Apartments":[{_id:"1",address:"Street 1"},{_id:"2",address:"Street 2"},{_id:"3",address:"Street 3"},{_id:"4",address:"Street 4"}],"Users":[{_id:"1",name:"User 1",favorites:["1","3"]},{_id:"2",name:"User 2",favorites:["2","3","4"]},{_id:"3",name:"User 3",favorites:[]}]}

db.Apartments.aggregate([{"$lookup":{"from":"Users","localField":"_id","foreignField":"favorites","as":"favoritesCount"}},{"$addFields":{"favoritesCount":{$size:"$favoritesCount"}}}])