db={"Users":[{"user_id":"SB4mKAxaBijQXnS73","username":"user1"},{"user_id":"SB4mdasd3ijQXnS73","username":"user2"}],"Views":[{"view_id ":"rhqioBkePzGCrRFLp","recipient_username":"user1"}]}

db.Users.aggregate([{"$lookup":{"from":"Views","localField":"username","foreignField":"recipient_username","let":{"usersUsername":"username"},"pipeline":[{"$match":{"$expr":{$ne:["$$usersUsername","$recipient_username"]}}},{$limit:1}],"as":"out"}},{$match:{out:{$ne:[]}}}])