title: backbone.js filter collection using substring
body: <p>The following problem is driving me crazy.</p>
  
  <pre><code>_.each(collection, function( account, key ){
  
      var totalPhy = that.physicianCollection.where({ 'Hospital_Id__c' : account.Id }).length;
      account.physicians = { 'total' : totalPhy };
  });
  </code></pre>
  
  <p>It is working when <code>Hospital_Id__c</code> same as account.Id. But my account Id is a sub string of the <code>hospital_Id__c</code>. How do I search and get the count? I tried index of and search methods. Pls suggest. Thanks in advance.</p>