Question

我有这样的Firebase结构：

ID为NMCQIE...的第一位用户有两位朋友。如何查询Firebase以获取第一个用户朋友的电子邮件？

Answer 1

final ArrayList<String> emails = new ArrayList<>();

    DatabaseReference dbref = FirebaseDatabase.getInstance().getReference().child("users").child(fuser.getUid()).child("friends");

    dbref.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            GenericTypeIndicator<Map<String, String>> gti = new GenericTypeIndicator<Map<String, String>>() {};

           // this contains the friend's list
            Map<String,String> friends = dataSnapshot.getValue(gti);

            Iterator<String> iterator = friends.keySet().iterator();

            while (iterator.hasNext()){

                // now for each friend fetch the email
      FirebaseDatabase.getInstance().getReference().child("users").child(iterator.next())
                        .child("email").addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        emails.add(dataSnapshot.getValue(String.class));
                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {

                    }
                });

            }




        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

为了提高效率（减少服务器调用），请尝试使用朋友列表存储电子邮件

Answer 2

假设您有ArrayList<String>名为friends，其中包含其firebase ID。然后你可以做类似的事情：

ArrayList<String> friendsMails = new ArrayList<>();

final ProgressDialog progressDialog = new ProgressDialog(Activity.this);
progressDialog.setMessage("Getting emails...");
progressDialog.setIndeterminate(true);
progressDialog.show();

DatabaseReference usersDBRef = FirebaseDatabase.getInstance().getReference();
usersDBRef.addListenerForSingleValueEvent(new ValueEventListener() {
   @Override
   public void onDataChange(DataSnapshot dataSnapshot) {
          String mail = String.valueOf(dataSnapshot.child("email").getValue(String.class));
          friendsMails.add(mail);
          if (friendsMails.size() == friends.size()){
              progressDialog.dismiss();
              //Do whatever you want with the list of mails
          }
   }

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

for(i = 0; i < friends.size(); i++){
    usersDBRef.child("users").child(friends.get(i));
}

查询firebase就像在哪里

2 个答案: