title: I want to calculate the sum of the ShoppingCart at Firebase with Angularfire2
body: <p>I'm building an experimental application that uses Angular2, Firebase and AngularFire2.
  This is what my data looks like:</p>
  
  <pre><code>{
    "ShoppingCartItem":
        {
          "item1":{
              "quantity": 2,
              "productId": "abc"
          },
          "item2":{
              "quantity": 1,
              "productId": "bcd"
          }
        }
    "Product"
        {
          "abc":{
              "name": "product1",
              "price": 5
      },
          "bcd":{
              "name": "product2",
              "price": 6
          }
      }
  }
  </code></pre>
  
  <p>Below are my cart.ts</p>
  
  <pre><code>this.items = this.af.database.list('ShoppingCartItem')
            .map(carts =&gt; {
              return carts.map(item =&gt; {
                item.product = this.af.database.object(`Product/${item.productId}`);
                return item;
              });
            });
  </code></pre>
  
  <p>Below are my cart.html</p>
  
  <pre><code>&lt;table&gt;
  &lt;tbody&gt;
      &lt;tr *ngFor="let item of (items | async)"&gt;
          &lt;td&gt;{{(item.product | async)?.name}}&lt;/td&gt;
          &lt;td&gt;&lt;input type="text" name="quantity" [(ngModel)]="item.quantity" size="1" class="form-control"&gt;&lt;/td&gt;
          &lt;td&gt;{{(item.product | async)?.price | number: '.2'}}&lt;/td&gt;
          &lt;td&gt;{{(item.product | async)?.price * item.quantity | number: '.2'}}&lt;/td&gt;
      &lt;/tr&gt;
  &lt;/tbody&gt;
  &lt;tfoot&gt;
      &lt;tr&gt;
        &lt;td colspan="2" class="text-right"&gt;
          &lt;strong&gt;Total :&lt;/strong&gt;
        &lt;/td&gt;
        &lt;td colspan="2" class="text-left"&gt;
          ?????????
        &lt;/td&gt;
      &lt;/tr&gt;
  &lt;/tfoot&gt;
  </code></pre>
  
  <p>
  I want to calculate the sum of the ShoppingCart. So I want to find the value of (2*5) + (1*6) = 16 as the data shows. How do i do it.</p>
  
  <p>This <a href="http://plnkr.co/edit/cWdXRGNk08NDtArDj10O" rel="nofollow noreferrer">plunker</a></p>