db={collA:[{_id:1,name:"Rec A"}],collB:[{refId:1,name:"Match A"},{refId:1,name:"Unmatch A"}]}

db.collA.aggregate([{$lookup:{from:"collB",localField:"_id",foreignField:"refId",as:"bs"}},{$project:{_id:1,bs:{"$filter":{"input":"$bs","cond":{$eq:["$$this.name","Match A"]}}}}}])