db={"a":[{some:"blablabla",b:ObjectId("5f79c15902db6c56be4720e4")}],"b":[{_id:ObjectId("5f79c15902db6c56be4720e4"),name:"bla",surname:"blabla"}]}

db.a.aggregate([{$match:{some:"blablabla"}},{"$lookup":{"from":"b","let":{"bId":"$b"},pipeline:[{$match:{$expr:{$and:[{$eq:["$$bId","$_id"]},{$eq:["$name","bla"]}]}}}],"as":"B"}},{$match:{"B.0":{$exists:true}}}])