title: Select all field values of an observable array in a comma separated string
body: <p>I've got an observableArray like this:</p>
  
  <pre><code>self.DisplayMessageCollection = ko.observableArray();
  </code></pre>
  
  <p>I'm getting a collection and pushing items to that <code>DisplayMessageCollection</code> like this: </p>
  
  <pre><code>self.DisplayMessageCollection.push({                            
                          messageid: msgid,
                          loader: 'block',
                          uploadopacity: 'uploadopacity',
                          sentstatus: 'Wait',
                          Chattype: self.ToChatType()
                      });
  </code></pre>
  
  <p>I need to get all the <code>messageid</code> field values as a comma seperated string from that array.</p>
  
  <p>I know I can <em>loop</em> all the items and get <code>messageid</code> values. But I want to know if we can query on the <code>observableArray</code>, i.e. somehow do a <em>single query</em> to get all the field values as a comma seperated string?</p>