如何检查数据库引用孩子是否存在

我正在聊天，想知道如何为一个对话设置一个ID。当你写信给某人时，id是“yourId + friendId”，所以如果你的朋友想回答你，他的对话Id应该仍然是“myId + hisId”。
我必须设置databasereference子id，但在此之前，我需要检查id是否存在“yourId + friendId”。我不知道如何。这里是我的代码：如何检查数据库引用孩子是否存在

databaseMessages= FirebaseDatabase.getInstance().getReference("conversation"); 
firebaseAuth=FirebaseAuth.getInstance(); 

if(databaseMessages.child(chattingUser.getUserId()+", "+firebaseAuth.getCurrentUser().getUid())!=null) { 
     databaseMessagesChild = databaseMessages.child(chattingUser.getUserId() + ", " + firebaseAuth.getCurrentUser().getUid()); 
    } 
else { 
     databaseMessagesChild = databaseMessages.child(firebaseAuth.getCurrentUser().getUid() + ", " + chattingUser.getUserId()); 
    }

它并没有工作。如何检查databaseMessages.child(chattingUser.getUserId()+", "+firebaseAuth.getCurrentUser().getUid())是否存在？

这里是我的全部代码：

public class Chat extends AppCompatActivity implements View.OnClickListener { 

private User chattingUser; 
FirebaseAuth firebaseAuth; 
DatabaseReference databaseMessages, databaseMessagesChild; 
ListView listViewConversation; 
List<Message> messageList; 


@Override 
public void onCreate(@Nullable Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.chat_screen); 

    databaseMessages= FirebaseDatabase.getInstance().getReference("conversation"); 
    firebaseAuth=FirebaseAuth.getInstance(); 

    Intent intent=getIntent(); 
    chattingUser= (User) intent.getSerializableExtra("User"); 
    TextView nazwa = (TextView)findViewById(R.id.chat_with); 
    nazwa.setText(chattingUser.getNickname()); 

    listViewConversation=(ListView)findViewById(R.id.list_conversation); 
    //listViewConversation.scrollTo(0, listViewConversation.getScrollY()); 
    messageList=new ArrayList<>(); 



    databaseMessagesChild = databaseMessages.child(firebaseAuth.getCurrentUser().getUid() + ", " + chattingUser.getUserId()); 


    DatabaseReference mref=databaseMessages.child(chattingUser.getUserId()+", "+firebaseAuth.getCurrentUser().getUid()); 
    mref.addValueEventListener(new ValueEventListener() { 
     public void onDataChange(DataSnapshot dataSnapshot) { 
      if (dataSnapshot.exists()){ 
       databaseMessagesChild = databaseMessages.child(chattingUser.getUserId() + ", " + firebaseAuth.getCurrentUser().getUid()); 
       Log.v("wybralem", "pierwszy host"); 
       Log.v("i ten klucz to:", databaseMessagesChild.getKey()); 

      } 
     } 
     @Override 
     public void onCancelled(DatabaseError databaseError) { 

     } 
    }); 








/* 
    if(databaseMessages.child(chattingUser.getUserId()+", "+firebaseAuth.getCurrentUser().getUid())!=null) { 
     databaseMessagesChild = databaseMessages.child(chattingUser.getUserId() + ", " + firebaseAuth.getCurrentUser().getUid()); 
     Log.v("wybralem", "pierwszy host"); 
    } 
    else { 
     databaseMessagesChild = databaseMessages.child(firebaseAuth.getCurrentUser().getUid() + ", " + chattingUser.getUserId()); 
     Log.v("wybralem", "drugi host"); 
    } */ 


    Button send = (Button)findViewById(R.id.send_button); 
    send.setOnClickListener(this); 

} 





@Override 
protected void onStart() { 
    super.onStart(); 
    Log.v("KEY",databaseMessagesChild.getKey()); 
    databaseMessagesChild.addValueEventListener(new ValueEventListener() { 
     @Override 
     public void onDataChange(DataSnapshot dataSnapshot) { 

      messageList.clear(); 

      for(DataSnapshot usersSnapshot : dataSnapshot.getChildren()){ 
       Message messages = usersSnapshot.getValue(Message.class); 
       messageList.add(messages); 

      } 

      MessagesList adapter = new MessagesList(Chat.this, messageList); 
      listViewConversation.setAdapter(adapter); 

     } 

     @Override 
     public void onCancelled(DatabaseError databaseError) { 

     } 
    }); 
} 






@Override 
public void onClick(View v) { 
    int i = v.getId(); 

    if(i==R.id.send_button){ 
     EditText message_field= (EditText) findViewById(R.id.message_field); 
     String message_text=message_field.getText().toString(); 
     message_field.setText(""); 
     String myId=firebaseAuth.getCurrentUser().getUid().toString(); 
     Message message = new Message(myId, chattingUser.getUserId() , message_text); 
     //Toast.makeText(Chat.this, firebaseAuth.getCurrentUser().getUid(), Toast.LENGTH_SHORT).show(); 

     databaseMessagesChild.push().setValue(message); 


    } 

}

}

来源

2017-08-03 Kuba Stonka

试试这个：

DatabaseReference mref=databaseMessages.child(chattingUser.getUserId()+", "+firebaseAuth.getCurrentUser().getUid()); 

mref.addValueEventListener(new ValueEventListener() { 
       @Override 
       public void onDataChange(DataSnapshot dataSnapshot) { 
        if (dataSnapshot.exists()){ 

        } 

       } 
       @Override 
       public void onCancelled(DatabaseError databaseError) { 

       } 
      });

来源

2017-08-03 13:13:26

但如何检查它，当我开始的意图只是一次onCrearte方法存在？我试过你的解决方案，它每次发送消息时都会检查。 –

你能告诉我你的oncreate（）方法吗？ –

当然，我添加了整个类 –

databaseReference.orderByChild("id").equalTo(yourId + friendId).addListenerForSingleValueEvent(new ValueEventListener() { 
    @Override 
    public void onDataChange(DataSnapshot dataSnapshot) { 
     if (dataSnapshot.exists()) { 
      // Data exists 
     } 
    } 

    @Override 
    public void onCancelled(DatabaseError databaseError) { 
    } 
});

来源

2017-08-03 13:14:30

如何检查数据库引用孩子是否存在

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