Firebase Android中的数据描述排序

Question

I am storing data in Firebase storage.

Object Comment with attribute timestamp . When I push data from device to Firebase I'm populating timestamp with currentTime and store in long data type.

When I do retrieving the data with firebaseRef.orderByChild("timestamp").limitToLast(15) result is not sorting how I expected.

I even played around with rules and no result:

{
    "rules": {
        ".read": true,
        ".write": true,
        ".indexOn": "streetrate",
        "streetrate": {
          ".indexOn": ".value"
        }
    }
}

I tried store timestamp in String data type, same issue.

Answer 1

Firebase can order the items in ascending order by a given property and then returns either the first N items ( limitToFirst() ) or the last N items ( limitToLast() ). There is no way to indicate that you want the items in descending order.

There are two options to get the behavior you want:

1. Use a Firebase query to get the correct data, then re-order it client-side

2. Add a field that has a descending value to the data

For the latter approach, it is common to have a inverted timestamp.

-1 * new Date().getTime();

Answer 2

one good solution I find if you are using recycler view to render that data...

mLayoutManager = new LinearLayoutManager(getActivity());
mLayoutManager.setReverseLayout(true);
mLayoutManager.setStackFromEnd(true);

// And set it to RecyclerView
mRecyclerView.setLayoutManager(mLayoutManager);

it will reverse the data rendering...

private static class ChatMessageViewHolder extends RecyclerView.ViewHolder {
         TextView messageText;
         TextView nameText;

         public ChatMessageViewHolder(View itemView) {
             super(itemView);
             nameText = (TextView)itemView.findViewById(android.R.id.text1);
             messageText = (TextView) itemView.findViewById(android.R.id.text2);
         }
     }

     FirebaseRecyclerViewAdapter<ChatMessage, ChatMessageViewHolder> adapter;
     ref = new Firebase("https://<yourapp>.firebaseio.com");

     RecyclerView recycler = (RecyclerView) 

     findViewById(R.id.messages_recycler);
         recycler.setHasFixedSize(true);

         //////////////////////////////////////////////////////////
         mLayoutManager = new LinearLayoutManager(getActivity());
         mLayoutManager.setReverseLayout(true);
         mLayoutManager.setStackFromEnd(true);
         recycler.setLayoutManager(mLayoutManager);
         /////////////////////////////////////////////////////////

         adapter = new FirebaseRecyclerViewAdapter<ChatMessage, ChatMessageViewHolder>(ChatMessage.class, android.R.layout.two_line_list_item, ChatMessageViewHolder.class, mRef) {
         public void populateViewHolder(ChatMessageViewHolder chatMessageViewHolder, ChatMessage chatMessage) {
             chatMessageViewHolder.nameText.setText(chatMessage.getName());
             chatMessageViewHolder.messageText.setText(chatMessage.getMessage());
         }
     };
     recycler.setAdapter(mAdapter);

Answer 3

I don't see any option to reverse the data. But One Brute way is to get the data.

List<ModelClass> mList=new ArrayList();
public void onDataChange(DataSnapshot dataSnapshot) 
{
     mList.clear();
     for(DataSnapshot children: dataSnapshot.getChildren()){
        ModelClass modelClass=children.getValue(ModelClass.class);
        mList.add(modelClass);
     }
     Collections.reverse(mList);
     Adapter.notifyDataSetChanged();
}

Answer 4

I have solved problem by extending FirebaseListAdapter and overriding getItem method:

public abstract class ReverseFirebaseListAdapter<T> extends FirebaseListAdapter<T>  {

public ReverseFirebaseListAdapter(Activity activity, Class<T> modelClass, int modelLayout, Query ref) {
    super(activity, modelClass, modelLayout, ref);
}

public ReverseFirebaseListAdapter(Activity activity, Class<T> modelClass, int modelLayout, DatabaseReference ref) {
    super(activity, modelClass, modelLayout, ref);
}

@Override
public T getItem(int position) {
  return super.getItem(getCount() - (position + 1));
}

}

Answer 5

The @Monet_z_Polski approach, based on

@Override
public T getItem(int position) {
  return super.getItem(getCount() - (position + 1));
}

does have a weird effect on not update FirebaseRecyclerView automatically (PopulateViewHolder is not triggered in realtime changes). So, the best option is use a negative key to index the data .

Answer 6

add a column namely timestamp with values (-1*System.currentTimeMillis()) and while fetching data use orderByChild("timestamp").limitToFirst(15) or orderByChild("timestamp").limitToLast(15) according to your requirement. it is smartway to get data in sorted manner, as firebase has no rule for descending order.

Answer 7

Sorting child items by TIMESTAMP can be done using android.support.v7.util.SortedList

  class post{
private Object time; 

public Object getTime() {
        return time;
    }

public void setTime(Object time) {
    this.time = time;
}
                        ...//rest code}

SortedList<post> data;

 data = new SortedList<post>(post.class, new SortedList.Callback<post>() {
        @Override
        public int compare(post o1, post o2) {
            Long o1l = Long.parseLong(o1.getTime().toString());
            Long o2l = Long.parseLong(o2.getTime().toString());

            return o2l.compareTo(o1l);
        }......//rest code


ref.addChildEventListener(new ChildEventListener() {
              @Override
              public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                 mSwipeRefreshLayout.setRefreshing(true);
                  post p=dataSnapshot.getValue(post.class);


                  data.add(p);



              }...// rest code

android.support.v7.util.SortedList can also be used with RecyclerView

Answer 8

If you are doing it on web then you can push values in an array and then print the stored values in reverse order.

var a=[]; //declaring an array a.
var num=snapshot.numChildren(); //storing number of children in var num.
a.push(snapshot.val()); // pushing data in the array.
if (a.length==num){ //checking if length of array is same as number of children.
a.reverse(); //reversing the array elements.
for(i=0; i<num; i++){
a[i].name;//this will sort the data in descending order.
}
}

Answer 9

Based on Himanshus answer but Im using kotlin and instead of the timestamp in the question Im ordering by score which should be similar. I'm not sure if firebase has an ascending or descending function but this is what ended working for me in my current project. I hope it will help someone else in the near future.

FirebaseFirestore.getInstance().collection("leaderboard")
            .orderBy("score")
            .addSnapshotListener { snapshot, exception ->

                if (exception != null) {
                    Log.e("Exception:", "Could not retrieve scores ${exception.localizedMessage}")
                }

                if (snapshot != null) {
                    scores.clear()

                    for (document in snapshot.documents) {
                        val data = document.data

                        val name = data?.get("name") as? String
                        val score = data?.get("score") as? Number

                        val documentId = document.id

                        val newScore = Score(name, score, documentId)
                        scores.add(newScore)
                    }

                    scores.reverse()
                    scoresAdapter.notifyDataSetChanged()
                }
            }

This is what worked for me. Happy coding!

Answer 10

This is a client side solution , it will sort the data according to a specific child in firebase database after filling the list

Collections.sort(list, new Comparator<ModuleClass>() {
                    @Override
                    public int compare(ModuleClass module, ModuleClass t1) {
                        return Integer.compare(module.getViews(), t1.getViews());
                    }
                });

Answer 11

If you are passing an arraylist to the recyclerView you can do something like this:

FirebaseDatabase.getInstance().getReference().child("parent_id")
.child("id").orderByChild("count");

    tpDBListener = tpDatabaseRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot snapshot) {
            tpUploads.clear();
            for(DataSnapshot postSnapshot : snapshot.getChildren()){
                newUpload pupload = postSnapshot.getValue(newUpload.class);
                pupload.setKey(postSnapshot.getKey());
                
                    tpUploads.add(pupload);//add new object on end
                    //tpUploads.add(0,pupload);//add new object at beginning
                
            }
            tpAdapter.notifyDataSetChanged();
        }

        @Override
        public void onCancelled(@NonNull DatabaseError error) {
            Toast.makeText(home.this, error.getMessage(),Toast.LENGTH_SHORT).show();
        }
    });

to flip the order so first is last and last is first I recommend when putting the value in the database putting it as 9999999999 and then -1 for what ever ordering you want then use the: if you are not ordeing them by a value or a child.

 tpUploads.add(0, pupload);//add new object at beginning

if you want to order by a value this is the better approach and initialize it in db as 9999999999 and then -1 for its value

.orderBy("child_ID") 

firebase will try to read each digit for example 1 is before 6 but 10 will also come before 6 because of the first digit. That is why I start off with 9999999999 as the starting value and decrease acordingly

Answer 12

public static List<'YourModel> desireList= new ArrayList<>();

//Your Realtime Database Query on "ORDERBY","Name" or Key or etc.

@Override
public void onDataChange(DataSnapshot dataSnapshot) 
{
 desireList.clear();
 List<YourModel>tempList = new ArrayList<>();//Create temp list Object.

 for (DataSnapshot dataSnapshotList : dataSnapshot.getChildren()) 
   {
    tempList.add(new YourModel(
       dataSnapshotList.child("Name").getValue() + "",
       dataSnapshotList.getKey() + "",
       dataSnapshotList.child("ORDERBY").getValue() + ""));
   }
       //Save object list in reverse order start.
       for (int i =tempList.size(); i>0; i--)
        {
          desireList.add(tempList.get(i-1));
        }
       //Save object list in reverse order end.

        tempList.clear(); //To clear memory only(not required.)
        //Then update your adapter
        yoursAdapter.notifyDataSetChanged();
 }

Answer 13

Sort at client side is simple and not require more system resources. Each data snapshot has previousChildkey field. If you want to desc sorting, imagine previousChildkey is nextChildKey. Here are my sample:

class LessonFirebaseArray<ObjectModel>{

  private ArrayList<ObjectModel> mItems;
  ...
  public LessonFirebaseArray() {
    mItems = new ArrayList<>();
  }

  public int addItem(ObjectModel item, boolean isReverse){
    int index;
    if (item.getPreviousChildKey() != null) {
      index = getIndexForKey(item.getPreviousChildKey());
      if (index < 0) {
        index = mItems.size();
      }else if(index>0 && !isReverse) {
        index = index + 1;      
      }
    }else{
      index = mItems.size();
    }
    mItems.add(index, item);
    notifyInsertedListeners(index);
    return index;
  }

  private int getIndexForKey(String key) {
    int index = 0;
    for (ObjectModel snapshot : mItems) {
      if (snapshot.getKey().equals(key)) {
         return index;
      } else {
         index++;
      }
    }
    return -1;
  }

  private void notifyInsertedListeners(int index) {
    if (mListener != null) {
      mListener.onInserted(index);
    }
  }
}

Answer 14

It is very simple,Use -1,-2,-3, etc to store data in the firebase database.Now the data will be displayed in the reverse order in recyclerView.

-3
-2
-1

Answer 15

您可以简单地反转您正在使用的列表（ListView 或 RecyclerView）。

Answer 16

As I understand there is no descending method , for default it is ascending only, but you can reverse it manually . For more information read an official documantation about sorting and filtering data of Realtime Database https://firebase.google.com/docs/database/android/lists-of-data#sorting_and_filtering_data

Answer 17

The Current version of firebase will allow you the fetch the data by sorting it in descending order.

All you have to do is pass an optional bool flag to let the firebase to know to fetch the data in descending order.

Syntax

Query<Object?> orderBy(Object field, {bool descending = false})

Example

QuerySnapshot eventsQuery =
        await ref.orderBy('DateTime', descending: true).get();

Thanks

Answer 18

Swift 3:

    let query = firebase.child(YourQueryPath).queryOrdered(byChild: YourQueryChild).queryLimited(toLast: YourLimit)

    query.observeSingleEvent(of: .value, with: { (snapshot) in
         // Reverse order here to get top **YourLimit** results in descending order
    })

Answer 19

I found the current firebase version supports descending order sorting:

citiesRef.orderBy("name", "desc").limit(3)

https://firebase.google.com/docs/firestore/query-data/order-limit-data

Hope it helps