firebase - get single object from list as snapshot

问题

So I have a list of objects stored in firebase and want to query for just one object using child equal to method but I am getting a map with single key value instead.

Code below showing what I want to achieve instead of dealing with hashmaps.

    final FirebaseDatabase database = FirebaseDatabase.getInstance();
    DatabaseReference coutryRef = database.getReference(FireContract.PATH_COUNTRY);

    Query countryQuery = coutryRef.orderByChild(FireContract.CHILD_NAME)
            .equalTo(TestUtilities.TEST_COUNTRY_ALGERIA).limitToFirst(1);

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

            Country country = dataSnapshot.getValue(Country.class);
            assertEquals(TestUtilities.TEST_COUNTRY_ALGERIA, country.getName());

            //get country key
            String coutryKey = dataSnapshot.getKey();   
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            fail("Failed to get country: " + databaseError.getMessage());

        }
    });

回答1:

When you fire a query, you will always get a list of results. Even when there is only a single item matching the query, the result will be a list of one item.

To handle the list, you can either use a ChildEvenListener or you can loop over the children in your ValueEventListener:

countryQuery.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for (DataSnapshot countrySnapshot: dataSnapshot.getChildren()) {

            Country country = countrySnapshot.getValue(Country.class);
            assertEquals(TestUtilities.TEST_COUNTRY_ALGERIA, country.getName());

            //get country key
            String countryKey = countrySnapshot.getKey(); 
        }  
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        fail("Failed to get country: " + databaseError.getMessage());
    }
});

来源：https://stackoverflow.com/questions/40247560/firebase-get-single-object-from-list-as-snapshot