android：如何通过引用另一个数据库引用属性

Question

我只想获得用户收藏夹中具有所需城市的项目。 例如，如果用户有20个收藏夹项，并且想要过滤特定城市中存在的收藏夹，该如何过滤呢？

对不起，我的英语。如果不清楚，请问我。

Answer 1

看看下面的数据库结构：

root : {
     user_01 : {
           fav_01 : {
                 data : val,
                 city : "city_01"
           }
           fav_02 : {
                 data : val,
                 city : "city_02"
           }
           fav_03 : {
                 data : val,
                 city : "city_01"
           }
     }
}

现在在andorid中进行这样的查询：

String user = "user_01";
String city_name = "city_01";
int limit = 10;
DatabaseReference favRef= FirebaseDatabase.getInstance().getReference("root/").child(user);
    Query query = favRef.orderByChild("city").equalTo(city_name).limitToFirst(limit);
    query.addListenerForSingleValueEvent(new ValueEventListener() {
         @Override
         public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
              Log.e(TAG, "onDataChange: "+dataSnapshot.toString());
              //you will get fav_01 and fav_3 objects
         }

         @Override
         public void onCancelled(@NonNull DatabaseError databaseError) {
              Log.e(TAG, "onCancelled: "+databaseError.toString());
         }
});

希望它可以帮助您 快乐的编码：）

Answer 2

package com.you.package.name;

import android.support.annotation.NonNull;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.Query;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;

/**
 * Created by harkal on 26-10-2018.
 */

public class Filter {

    private static final String TAG = "";

    private String city;
    private String user;

    private OnFoodDataCallBack onFoodDataCallBack;

    public void setOnFoodDataCallBack(OnFoodDataCallBack onFoodDataCallBack) {
        this.onFoodDataCallBack = onFoodDataCallBack;
    }

    public Filter(String city, String user) {
        this.city = city;
        this.user = user;
    }

    public void init(){
        DatabaseReference userFavRef = FirebaseDatabase.getInstance().getReference("Path/To/Users/");
        userFavRef.child(user).child("favourites").addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                if(dataSnapshot.exists()){
                    //now get the favs with the filtered city
                    int initKey = 0;
                    final int totalFavs = (int)dataSnapshot.getChildrenCount();
                    final ArrayList<String> foodsWithCity = new ArrayList<>();
                    for(DataSnapshot snapshot : dataSnapshot.getChildren()){

                        initKey = initKey+1;

                        String foodKey = snapshot.getKey();//this is the key for every fav food 

                        //now query all the food nodes with the city

                        DatabaseReference foodRef = FirebaseDatabase.getInstance().getReference("Path/To/Foods/");
                        Query query = foodRef.child(foodKey).orderByChild("city").equalTo(city);
                        final int finalInitKey = initKey;
                        query.addListenerForSingleValueEvent(new ValueEventListener() {
                            @Override
                            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                                if(dataSnapshot.exists()){
                                    //there is a food item with the queried city

                                    foodsWithCity.add(dataSnapshot.getKey());

                                    if(finalInitKey == totalFavs){
                                        if(onFoodDataCallBack != null){
                                            onFoodDataCallBack.onSuccess(foodsWithCity);
                                        }
                                    }                                    
                                }else{
                                    //there is no food with this city
                                    if(finalInitKey == totalFavs){
                                        if(onFoodDataCallBack != null){
                                            onFoodDataCallBack.onFailed("there is no food with this city");
                                        }
                                    }
                                }
                            }

                            @Override
                            public void onCancelled(@NonNull DatabaseError databaseError) {
                                if(onFoodDataCallBack != null){
                                    onFoodDataCallBack.onFailed(databaseError.toString());
                                }
                            }
                        });

                    }

                }else{
                    if(onFoodDataCallBack != null){
                        onFoodDataCallBack.onFailed("no fav data about user");
                    }
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                if(onFoodDataCallBack != null){
                    onFoodDataCallBack.onFailed(databaseError.toString());
                }
            }
        });
    }

    public interface OnFoodDataCallBack{

        void onSuccess(ArrayList<String> foodWithCities);
        void onFailed(String error);

    }

}

您可以通过以下方式使用代码：

Filter filter = new Filter("washingaton", "user_01");
        filter.setOnFoodDataCallBack(new Filter.OnFoodDataCallBack() {
            @Override
            public void onSuccess(ArrayList<String> foodWithCities) {
                //here is the data you wanted
            }

            @Override
            public void onFailed(String error) {
                //handle the error and change ui accordingly
                Log.e(TAG, "onFailed: "+error);
            }
        });
        filter.init();

希望有帮助

快乐的编码：）