title: How to make firebase data read only once in typescript
body: <pre><code>readProductFromServer() {
  
      this.qS = this.afDatabase.list('product_table', ref =&gt; {
  
        return ref.limitToLast(1000) ;
  
  
      }).snapshotChanges().map(changes =&gt; {
        return changes.map(c =&gt; ({ key1: c.payload.key, ...c.payload.val() }));
      });
      this.qS.subscribe(values =&gt; {
  
      });
    }
  </code></pre>
  
  <p>How to make this code read only once. I searched on the internet and not get an answer to my code, some solution is available in StackOverflow  but not match  with my code?</p>