java - orderByChild 查询不适用于 addListenerForSingleValueEvent Firebase

Question

我尝试按子值“月”对数据库进行排序，但 orderByChild 不起作用。当我使用 onChildEventListener 而不是 addListenerForSingleValueEvent 时它可以工作但我不想使用 childEventListener

这是我的数据库

"Stats" : {
    "2019" : {
      "YxdZZHGy3rW4xdjORfk2i5mFRYG2" : {
        "August" : {
          "Weight" : {
            "Body_Fat" : "29",
            "Month" : 8,
            "Weight" : "68"
          }
        },
        "October" : {
          "Weight" : {
            "Body_Fat" : "29",
            "Month" : 10,
            "Weight" : "67"
          }
        },
        "September" : {
          "Weight" : {
            "Body_Fat" : "28.5",
            "Month" : 9,
            "Weight" : "65.5"
          }
        }
      }
    }
},

这是java代码

 userStatsDatabase.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            if (dataSnapshot.exists()) {
                for (DataSnapshot ds : dataSnapshot.getChildren()) {
                    Query query = userStatsDatabase.child(ds.getKey()).child("Weight").orderByChild("Month").limitToFirst(12);
                    query.addListenerForSingleValueEvent(new ValueEventListener() {
                        @Override
                        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                            if (dataSnapshot.exists()) {
                                Map<String, Object> map = (Map<String, Object>) dataSnapshot.getValue();
                                pastMonthsBodyFat = Float.parseFloat(map.get("Body_Fat").toString());
                                pastMonthsWeight = Float.parseFloat(map.get("Weight").toString());

                                pastWeightList.add(pastMonthsWeight);
                                pastBodyFatList.add(pastMonthsBodyFat);
                                Toast.makeText(Stats.this, map.get("Month").toString(), Toast.LENGTH_SHORT).show();
                            }
                        }

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

                        }
                    });
                }

Toast 应该按顺序打印月份值 8,9,10，但它正在打印值 8,10,9

Answer 1

如果userStatsDatabase指向此JSON(对于特定用户):

  {
    "August" : {
      "Weight" : {
        "Body_Fat" : "29",
        "Month" : 8,
        "Weight" : "68"
      }
    },
    "October" : {
      "Weight" : {
        "Body_Fat" : "29",
        "Month" : 10,
        "Weight" : "67"
      }
    },
    "September" : {
      "Weight" : {
        "Body_Fat" : "28.5",
        "Month" : 9,
        "Weight" : "65.5"
      }
    }
  }

然后您可以通过以下方式获取按月顺序排列的结果:

Query query = userStatsDatabase..orderByChild("Weight/Month");
query.addListenerForSingleValueEvent(new ValueEventListener() {
  @Override
  public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
    for (DataSnapshot monthSnapshot: dataSnapshot.getChildren()) {
      System.out.println(monthSnapshot.getKey()); // August, September, October
      System.out.println(monthSnapshot.child("Weight/Month").getValue(Long.class)); // 8, 9, 10
    }
  }
  ...