java - 无法在 FirebaseRecyclerView 中显示子节点数据。数据库异常: Can't convert object to class name

Question

我需要显示一个列表(来自 Firebase 实时数据库的数据绑定(bind)到 RecyclerView)，该列表位于 root -> History -> UID -> location, date 中。我目前只能显示历史记录下的所有数据。当我尝试将History节点过滤到UID的子节点时出现问题，并给出以下错误:

com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type com.example.ModelHistory

这是我的 ModelHistory.class:

public class ModelHistory {

String location, date;

// Constructor
public ModelHistory() {

}

public String getLocation() {
    return location;
}
// It shows a warning here that setLocation is never used
public void setLocation(String location) {
    this.location = location;
}

public String getDate() {
    return date;
}

public void setDate(String date) {
    this.date = date;
}
}

这是我的RecentHistory.java:

public class RecentHistory extends AppCompatActivity {

    RecyclerView mRecyclerView;
    FirebaseDatabase mFirebaseDatabase;
    DatabaseReference mRef, uidRef;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_recent_history);

        // Set ActionBar
        ActionBar actionBar = getSupportActionBar();
        actionBar.setTitle("History");

        mRecyclerView = findViewById(R.id.rvHistory);
        mRecyclerView.setHasFixedSize(true);


        // Set Layout
        mRecyclerView.setLayoutManager(new LinearLayoutManager(this));


        // Send Query to Firebase Db
        mFirebaseDatabase = FirebaseDatabase.getInstance();
        String currentuser = FirebaseAuth.getInstance().getCurrentUser().getUid();

        mRef = mFirebaseDatabase.getReference("History");
        uidRef = mRef.child(currentuser);
        Log.d("TAG", uidRef.toString());

    }

    @Override
    protected void onStart() {
        super.onStart();
        FirebaseRecyclerAdapter<ModelHistory, ViewHolder> firebaseRecyclerAdapter =
                new FirebaseRecyclerAdapter<ModelHistory, ViewHolder>(
                        ModelHistory.class,
                        R.layout.historyrow,
                        ViewHolder.class,
                        mRef
                        //uidRef  //  Error when I replace this with mRef
                ) {
                    @Override
                    protected void populateViewHolder(ViewHolder viewHolder, ModelHistory model, int position) {
                        viewHolder.setHistoryDirectory(getApplicationContext(), model.getLocation(), model.getDate());
                    }

                    @Override
                    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
                        ViewHolder viewHolder = super.onCreateViewHolder(parent, viewType);
                        return viewHolder;
                    }
                };
        // Set adapter to recycler view
        mRecyclerView.setAdapter(firebaseRecyclerAdapter);

    }
}

如果需要xml，请告诉我，我将编辑它。

Answer 1

您收到以下错误:

com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type com.example.ModelHistory

因为当您传递 uidRef 时，这是正确的行为到FirebaseRecyclerAdapter构造函数。

I'm currently only able to display all the data under History.

发生这种情况是因为您的适配器被定义为 FirebaseRecyclerAdapter<ModelHistory, ViewHolder> ，这意味着它应该加载 ModelHistory 类型的所有对象它在您的 mRef 下找到引用一下，其实是正确的。该引用下存在的所有对象的类型均为 ModelHistory .

The problem occurs when I try to filter out the History node to the child node of UID

您不能简单地更改引用并期望以这种方式过滤结果，因为您的适配器仍然是 ModelHistory 类型。并且不是 String 类型。所以当使用uidRef时引用，这意味着您正在尝试加载 ModelHistory 类型的所有对象在该引用之下，显然不退出。下root -> History -> UID仅存在 String 类型的属性 ( location and date )，这就是您收到该错误的原因。

如果您想过滤该数据，您应该创建一个如下所示的查询:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference hisotryRef = rootRef.child("History");
Query query = hisotryRef.orderByChild("location").equalTo("Test");

结果在你的 RecyclerView 中只会有一项，在本例中是最后一项，即具有包含 Test 的 location 属性的一项.

除此之外，您正在使用旧版本的 Firebase-UI Library 。我建议您更新到最新版本。