Screen 1 Outline of Firebase database management.
this.itemsRef = this.getRef().child('items’);　For Event Class reference pointer
this.userRef = this.getRef().child('user’);　　For User Registration  Class

getRef() {return firebaseApp.database().ref();}

Screen 1 Outline of Firebase database management.
this.itemsRef = this.getRef().child('items’);　For Event Class reference pointer
this.userRef = this.getRef().child('user’);　　For User Registration  Class

getRef() {return firebaseApp.database().ref();}

render section
     mail=this.state.user.email;
     console.log('mail = '  + mail);
     mail=mail.substring(0,mail.indexOf("@"));
     console.log('after modify mail = ' + mail);
    this.userRef = firebaseApp.database().ref('user/' + mail);
    this.userRef.push(new User(event,password,user);
This screen was OK and DB was created as follows.
rera06x
*
      -KXih12q_2XE24UQ6S7p
    *
event:
      *
password:
*
user
    *
_key:
    *

Screen ２  for Receptionist  and Firebase database process's outline
var user = firebase.auth().currentUser;　 same as screen 1, to get     user account

componentDidMount() {
this.listenForItems(this.userRef);

  this.userRef = firebaseApp.database().ref('user/' + mail);

listenForItems(userRef) {　　　　　　　In this section, The Registration    list was get,
userRef.on('value', (snap) => {