firebase documents
Users' Table| uid | photoURL | displayName |            
| jackass | avatar.png | James Bond 

Blogs' Table| id | title | uid | 

//javascript
blog.service.ts
file getPages(){ const item: AngularFirestoreCollection = this.afs.collection(`pages`, (ref) => ref.orderBy('createdAt', 'desc'));     
 return item.valueChanges().pipe(switchMap(arr => {      
  const userObservable = arr.map(user => this._afs.doc(`users/${user.uid}`)    .valueChanges().pipe(first()) );      
  return combineLatest(...userObservable) .pipe( map((...users) => {      
  arr.forEach((author, index) => {
    author['author_avatar'] = users[0][index]['photoURL'];                  
    author['author_username'] = users[0][index]['displayName'];      
  });              
  return arr;    
  })    
 );  
 }));      
}