在React-Native状态下对数组进行排序

Question

我一直试图按日期对数组（schoolClasses）中的对象进行排序，但没有成功。

我使用Firebase获得所有课程，并且，对于每个课程，我都使用Firebase获取其主题名称。

然后我按日期对数组进行排序并调用setState，传递已排序的数组：

export default class SchoolClassesScreen extends React.Component {
  constructor() {
    super();
    this.state = {
      schoolClasses: [],
      currentClass: {}
    };
  }

  componentDidMount() {
    FirebaseDatabase.database().ref('schoolClasses').once('value').then(snapshot => {
      let schoolClasses = snapshot.val();
      Object.keys(schoolClasses).forEach(key => {
        let schoolClass = schoolClasses[key];
        FirebaseDatabase.database().ref('subject/' + schoolClass.subject).once('value').then(snapshot => {
          schoolClass.subject = snapshot.val().name;
          let schoolClasses = this.state.schoolClasses.slice();
          schoolClasses.push(schoolClass);
          schoolClasses.sort((sc1, sc2) => new Date(sc1.date) - new Date(sc2.date));
          this.setState({schoolClasses: schoolClasses, currentClass: schoolClasses[0]});
        });
      });
    });
  }

  render() {
    // ...
  }
}

排序与否并没有任何区别。数组总是以相同的顺序。

我错过了什么？感谢。

更新1

数据库结构：

{
  "schoolClasses" : {
    "schoolClass1" : {
      "date" : "2017-11-07 18:45:00",
      "subject" : "security"
    },
    "schoolClass3" : {
      "date" : "2017-11-09 18:45:00",
      "subject" : "database1"
    },
    "schoolClass2" : {
      "date" : "2017-11-08 18:45:00",
      "subject" : "database1"
    }
  },
  "subject" : {
    "security" : {
      "name" : "I.T. Security"
    },
    "database1" : {
      "name" : "Database I"
    }
  }
}