db={members:[{_id:"62692f3503ed123e35a56a12",name:"John",}],emails:[{_id:"74331a2663ed999e35a33a09",memberId:"62692f3503ed123e35a56a12",email:"john@john.com"}]}

db.members.aggregate([{"$lookup":{from:"emails",localField:"_id",foreignField:"memberId",as:"emails"}},{$addFields:{emails:{$map:{input:"$emails",in:"$$this.email"}}}}])