db={"pipedrive":[{_id:1,deal_id:25698},{_id:2,deal_id:45879},{_id:3,deal_id:54142}],"leads":[{_id:1,name:"Teste A",deals_id:[25698,45879]},{_id:2,name:"Teste B",deals_id:[54142]},{_id:3,name:"Teste B",/** deals_id missing*/}]}

db.pipedrive.aggregate([{$lookup:{from:"leads",localField:"deal_id",foreignField:"deals_id",as:"Results"}},{$unwind:"$Results"},{$project:{deal_id:true,name:"$Results.name"}}])