db={parent:[{_id:1},{_id:2},{_id:3}],child1:[{_id:1,child1:"a",parent:1},{_id:2,child1:"b",parent:1},{_id:3,child1:"c",parent:2}],child2:[{_id:1,child1:"d",parent:2},{_id:2,child1:"e",parent:3}]}

db.parent.aggregate([{"$lookup":{"from":"child1","localField":"_id","foreignField":"parent","as":"child1"}},{"$lookup":{"from":"child2","localField":"_id","foreignField":"parent","as":"child2"}}])