Work with Lists of Data on Android

This document covers working with lists of data in Firebase. To learn the basics of reading and writing Firebase data see Read and Write Data on Android.

Get a DatabaseReference

To read and write data from the database, you need an instance of DatabaseReference:

private DatabaseReference mDatabase;
// ...
mDatabase = FirebaseDatabase.getInstance().getReference();

Read and write lists

Append to a list of data

Use the push() method to append data to a list in multiuser applications. The push() method generates a unique key every time a new child is added to the specified Firebase reference. By using these auto-generated keys for each new element in the list, several clients can add children to the same location at the same time without write conflicts. The unique key generated by push() is based on a timestamp, so list items are automatically ordered chronologically.

You can use the reference to the new data returned by the push() method to get the value of the child's auto-generated key or set data for the child. Calling getKey() on a push() reference returns the value of the auto-generated key.

You can use these auto-generated keys to simplify flattening your data structure. For more information, see the data fan-out example.

Listen for child events

When working with lists, your application should listen for child events rather than the value events used for single objects.

Child events are triggered in response to specific operations that happen to the children of a node from an operation such as a new child added through the push() method or a child being updated through the updateChildren() method. Each of these together can be useful for listening to changes to a specific node in a database.

In order to listen for child events on DatabaseReference, attach a ChildEventListener:

Listener Event callback Typical usage
ChildEventListener onChildAdded() Retrieve lists of items or listen for additions to a list of items. This callback is triggered once for each existing child and then again every time a new child is added to the specified path. The DataSnapshot passed to the listener contains the the new child's data.
onChildChanged() Listen for changes to the items in a list. This event fired any time a child node is modified, including any modifications to descendants of the child node. The DataSnapshot passed to the event listener contains the updated data for the child.
onChildRemoved() Listen for items being removed from a list. The DataSnapshot passed to the event callback contains the data for the removed child.
onChildMoved() Listen for changes to the order of items in an ordered list. This event is triggered whenever the onChildChanged() callback is triggered by an update that causes reordering of the child. It is used with data that is ordered with orderByChild or orderByValue.

For example, a social blogging app might use these methods together to monitor activity in the comments of a post, as shown below:

ChildEventListener childEventListener = new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String previousChildName) {
        Log.d(TAG, "onChildAdded:" + dataSnapshot.getKey());

        // A new comment has been added, add it to the displayed list
        Comment comment = dataSnapshot.getValue(Comment.class);

        // ...
    }

    @Override
    public void onChildChanged(DataSnapshot dataSnapshot, String previousChildName) {
        Log.d(TAG, "onChildChanged:" + dataSnapshot.getKey());

        // A comment has changed, use the key to determine if we are displaying this
        // comment and if so displayed the changed comment.
        Comment newComment = dataSnapshot.getValue(Comment.class);
        String commentKey = dataSnapshot.getKey();

        // ...
    }

    @Override
    public void onChildRemoved(DataSnapshot dataSnapshot) {
        Log.d(TAG, "onChildRemoved:" + dataSnapshot.getKey());

        // A comment has changed, use the key to determine if we are displaying this
        // comment and if so remove it.
        String commentKey = dataSnapshot.getKey();

        // ...
    }

    @Override
    public void onChildMoved(DataSnapshot dataSnapshot, String previousChildName) {
        Log.d(TAG, "onChildMoved:" + dataSnapshot.getKey());

        // A comment has changed position, use the key to determine if we are
        // displaying this comment and if so move it.
        Comment movedComment = dataSnapshot.getValue(Comment.class);
        String commentKey = dataSnapshot.getKey();

        // ...
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        Log.w(TAG, "postComments:onCancelled", databaseError.toException());
        Toast.makeText(mContext, "Failed to load comments.",
                Toast.LENGTH_SHORT).show();
    }
};
ref.addChildEventListener(childEventListener);

Listen for value events

While using a ChildEventListener is the recommended way to read lists of data, there are situations where attaching a ValueEventListener to a list reference is useful.

Attaching a ValueEventListener to a list of data will return the entire list of data as a single DataSnapshot, which you can then loop over to access individual children.

Even when there is only a single match for the query, the snapshot is still a list; it just contains a single item. To access the item, you need to loop over the result:

// My top posts by number of stars
myTopPostsQuery.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
            // TODO: handle the post
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        // Getting Post failed, log a message
        Log.w(TAG, "loadPost:onCancelled", databaseError.toException());
        // ...
    }
});

This pattern can be useful when you want to fetch all children of a list in a single operation, rather than listening for additional onChildAdded events.

Detach listeners

Callbacks are removed by calling the removeEventListener() method on your Firebase database reference.

If a listener has been added multiple times to a data location, it is called multiple times for each event, and you must detach it the same number of times to remove it completely.

Calling removeEventListener() on a parent listener does not automatically remove listeners registered on its child nodes; removeEventListener() must also be called on any child listeners to remove the callback.

Sorting and filtering data

You can use the Realtime Database Query class to retrieve data sorted by key, by value, or by value of a child. You can also filter the sorted result to a specific number of results or a range of keys or values.

Sort data

To retrieve sorted data, start by specifying one of the order-by methods to determine how results are ordered:

Method Usage
orderByChild() Order results by the value of a specified child key.
orderByKey() Order results by child keys.
orderByValue() Order results by child values.

You can only use one order-by method at a time. Calling an order-by method multiple times in the same query throws an error.

The following example demonstrates how you could retrieve a list of a user's top posts sorted by their star count:

// My top posts by number of stars
String myUserId = getUid();
Query myTopPostsQuery = databaseReference.child("user-posts").child(myUserId)
        .orderByChild("starCount");
myTopPostsQuery.addChildEventListener(new ChildEventListener() {
    // TODO: implement the ChildEventListener methods as documented above
    // ...
});

This defines a query that when combined with a child listener synchronizes the client with the user's posts from the path in the database based on their user ID, ordered by the number of stars each post has received. This technique of using IDs as index keys is called data fan out, you can read more about it in Structure Your Database.

The call to the orderByChild() method specifies the child key to order the results by. In this case, posts are sorted by the value of the "starCount" child in each post. For more information on how other data types are ordered, see How query data is ordered.

Filtering data

To filter data, you can combine any of the limit or range methods with an order-by method when constructing a query.

Method Usage
limitToFirst() Sets the maximum number of items to return from the beginning of the ordered list of results.
limitToLast() Sets the maximum number of items to return from the end of the ordered list of results.
startAt() Return items greater than or equal to the specified key or value depending on the order-by method chosen.
endAt() Return items less than or equal to the specified key or value depending on the order-by method chosen.
equalTo() Return items equal to the specified key or value depending on the order-by method chosen.

Unlike the order-by methods, you can combine multiple limit or range functions. For example, you can combine the startAt() and endAt() methods to limit the results to a specified range of values.

Even when there is only a single match for the query, the snapshot is still a list; it just contains a single item. To access the item, you need to loop over the result:

// My top posts by number of stars
myTopPostsQuery.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
            // TODO: handle the post
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        // Getting Post failed, log a message
        Log.w(TAG, "loadPost:onCancelled", databaseError.toException());
        // ...
    }
});

Limit the number of results

You can use the limitToFirst() and limitToLast() methods to set a maximum number of children to be synced for a given callback. For example, if you use limitToFirst() to set a limit of 100, you initially only receive up to 100 onChildAdded() callbacks. If you have fewer than 100 items stored in your Firebase database, an onChildAdded() callback fires for each item.

As items change, you receive onChildAdded() callbacks for items that enter the query and onChildRemoved() callbacks for items that drop out of it so that the total number stays at 100.

The following example demonstrates how example blogging app defines a query to retrieve a list of the 100 most recent posts by all users:

// Last 100 posts, these are automatically the 100 most recent
// due to sorting by push() keys
Query recentPostsQuery = databaseReference.child("posts")
        .limitToFirst(100);

This example only defines a query, to actually synchronize data it needs to have an attached listener.

Filter by key or value

You can use startAt(), endAt(), and equalTo() to choose arbitrary starting, ending, and equivalence points for queries. This can be useful for paginating data or finding items with children that have a specific value.

How query data is ordered

This section explains how data is sorted by each of the order-by methods in the Query class.

orderByChild

When using orderByChild(), data that contains the specified child key is ordered as follows:

  1. Children with a null value for the specified child key come first.
  2. Children with a value of false for the specified child key come next. If multiple children have a value of false, they are sorted lexicographically by key.
  3. Children with a value of true for the specified child key come next. If multiple children have a value of true, they are sorted lexicographically by key.
  4. Children with a numeric value come next, sorted in ascending order. If multiple children have the same numerical value for the specified child node, they are sorted by key.
  5. Strings come after numbers and are sorted lexicographically in ascending order. If multiple children have the same value for the specified child node, they are ordered lexicographically by key.
  6. Objects come last and are sorted lexicographically by key in ascending order.

orderByKey

When using orderByKey() to sort your data, data is returned in ascending order by key.

  1. Children with a key that can be parsed as a 32-bit integer come first, sorios/read-and-write#listen_for_value_eventsted in ascending order.
  2. Children with a string value as their key come next, sorted lexicographically in ascending order.

orderByValue

When using orderByValue(), children are ordered by their value. The ordering criteria are the same as in orderByChild(), except the value of the node is used instead of the value of a specified child key.

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