title: Ionic 2 update specific child with AngularFire2
body: <p>I want to update specific data in Firebase, 2 cases:</p>
  
  <ol>
  <li>I want to update specific child as shows in this photo:</li>
  </ol>
  
  <p><a href="https://i.stack.imgur.com/T3mLV.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/T3mLV.jpg" alt="enter image description here"></a></p>
  
  <ol start="2">
  <li>i have list of courses that chosen with ion-select in ionic 2 and i want to update\push each of them below the courses key
  for example:</li>
  </ol>
  
  <p>in firebase it will be like that:</p>
  
  <p>courses:{
       0: course1,
       1: course2,
       2:course3
  };</p>
  
  <p>I tried this code but it's not works</p>
  
  <pre><code>select_courseFun(course_array:any){
      var counter=0;
      for(let c of course_array){
          counter++;
          this.coursesPref.update(counter.toString(),{
          name: c
          });
      }
  }
  </code></pre>