db={"a":[{"_id":"a1","join_field":33}],"b":[{"_id":"b1","join_field":[33,22]}],"meta":[{"by":"steevej","at":"gmail"}]}

db.a.aggregate([{"$lookup":{"from":"b",let:{"jf":"$join_field"},pipeline:[{$match:{$expr:{$in:["$$jf","$join_field"]}}}],as:"lookup"}}])