db={"event":[{"_id":1,"eventActivity":[1]},{"_id":2,"eventActivity":[2,3]},{"_id":3,"eventActivity":[4]}],"event-activity":[{"_id":1,"activity":1},{"_id":2,"activity":4}]}

db.event.aggregate([{"$lookup":{"from":"event-activity","as":"activities","let":{"ea":"$eventActivity"},"pipeline":[{"$match":{"$expr":{"$in":["$activity","$$ea"]}}}]}}])