db={"collA":[{id:2,name:"test"},{id:4,name:"test4"}],"collB":[{id:444,name:"a",colA_id:2},{id:555,name:"b",colA_id:2},{id:555,name:"c",colA_id:10}]}

db.collA.aggregate([{"$lookup":{"from":"collB","localField":"id","foreignField":"colA_id","as":"list_of_b"}}])