[{"_id":ObjectId("5716617f4af77ca97a9614bd"),"flag":{"status":true,"userId":ObjectId("606b0e9f00bece43471baa50")},"text":"Main Comment 1","reply":[{"_id":ObjectId("571661cd4af77ca97a9614c1"),"flag":{"status":false},"text":"Main Comment 1 replay 1"},{"_id":ObjectId("571661cd4af77ca97a9614c2"),"flag":{"status":true,"userId":ObjectId("606b0e9f00bece43471baa48")},"text":"Main Comment 1 replay 2"}]}]

db.collection.aggregate([{$match:{$or:[{"flag.status":true},{"reply.flag.status":true}]}},{$unwind:{path:"$reply",preserveNullAndEmptyArrays:true}},{$project:{_id:1,text:1,reply:{$cond:[{$eq:["$reply.flag.status",true]},{_id:"$reply._id",text:"$reply.text"},{}]}}}])