聚合$ push + $ lookup

Question

我需要在数组中填充...

messageModel.aggregate([
        {"$match": {$and: [
                    {$or: [{"to": system.mongoose.Types.ObjectId(userId)}, {"from": system.mongoose.Types.ObjectId(userId)}]}
                ]
            }
        },

        {
            "$group": {
                "_id": "$conversationId",
                "conversation": {"$push": {'from': "$from"}}
            }

        },
        {"$lookup": {
                "from": "user",
                "localField": "conversation.from",
                "foreignField": "_id",
                "as": "to"
            }},
        {"$unwind": {"path": "$conversation.from"}},
        {"$project": {
                "conversation.from.firstName": "$conversation.from.firstName"
            }
        }
    ]

这个返回空数组有什么问题？如果我不使用数组，那么它可以工作，如果只是采用第一个元素......任何解决方案？