db={users:[{_id:ObjectId("65849c075dc62111f508569a"),"name":"bob","username":"bob123"},{_id:ObjectId("65849c075dc62111f508569b"),"name":"sally","username":"sally123"}],orders:[{_id:ObjectId("65440e4950b540cd1d628e82"),"userId":ObjectId("65849c075dc62111f508569a"),"products":["a","b","c"],"amount":5},{_id:ObjectId("65440e4950b540cd1d628e83"),"userId":ObjectId("65849c075dc62111f508569a"),"products":["d","e","f"],"amount":6},{_id:ObjectId("65440e4950b540cd1d628e84"),"userId":ObjectId("65849c075dc62111f508569b"),"products":["q","r","s"],"amount":10},{_id:ObjectId("65440e4950b540cd1d628e85"),"userId":ObjectId("65849c075dc62111f508569b"),"products":["x","y","z"],"amount":12}]}

db.orders.aggregate([{$group:{"_id":"$userId","totalAmount":{$sum:"$amount"}}},{$lookup:{"from":"users","localField":"_id","foreignField":"_id","as":"user"}},{$unwind:"$user"},{$project:{"name":"$user.name","username":"$user.username","totalAmount":1}}])