我正在使用Kotlin和Firebase开发一个Android原生应用程序，我有一个名为Topic的集合和2个类型引用字段，一个用于User，另一个用于Category，我试图通过引用文档获取所有文档，但似乎不起作用：

db.collection("topic").get().addOnSuccessListener { result ->
        for (document in result) {
            Log.e("success", "${document.id} => ${document.data.get("subject")}")
            var topic: Topic = Topic(
                document.id as String,
                document.data.get("subject") as String,
                document.data.get("content") as String,
                document.data.get("created_at") as String,
                document.getDocumentReference("Category") as Category,
                document.getDocumentReference("User") as User
            )}

字符串
没有错误，但在我的打印（主题）中也没有结果。
这是主题类

class Topic : Serializable {
    var id : String = ""
    var subject : String = ""
    var content : String = ""
    var created_at : String = ""
    var cat = Category()
    var user = User()

    constructor(){}

    constructor(
        id: String,
        subject: String,
        content: String,
        created_at: String,
        cat: Category,
        user: User
    ) {
        this.id = id
        this.subject = subject
        this.content = content
        this.created_at = created_at
        this.cat = cat
        this.user = user
    }

    override fun toString(): String {
        return "Topic(id='$id', subject='$subject', content='$content', created_at='$created_at', cat=$cat, user=$user)"
    }
}

型

的数据