如何在Firebase实时数据库中迭代多个按键的子项？

var database = firebase.database();

- your-database
  - keys
    - key1
      - child1: value1
      - child2: value2
    - key2
      - child1: value3
      - child2: value4

database.ref('keys').once('value').then(function(snapshot) {
  snapshot.forEach(function(childSnapshot) {
    var key = childSnapshot.key;
    var childData = childSnapshot.val();
    console.log("Key: " + key);
    console.log("Child Data: " + JSON.stringify(childData));
  });
});

database.ref('keys').on('value', function(snapshot) {
  snapshot.forEach(function(childSnapshot) {
    var key = childSnapshot.key;
    var childData = childSnapshot.val();
    console.log("Key: " + key);
    console.log("Child Data: " + JSON.stringify(childData));
  });
});