title: create a json string from NSArray
body: <p>In my iPhone aplication I have a list of custom objects. I need to create a json string from them. How I can implement this with SBJSON or iPhone sdk?</p>
  
  <pre><code> NSArray* eventsForUpload = [app.dataService.coreDataHelper fetchInstancesOf:@"Event" where:@"isForUpload" is:[NSNumber numberWithBool:YES]];
      SBJsonWriter *writer = [[SBJsonWriter alloc] init];  
      NSString *actionLinksStr = [writer stringWithObject:eventsForUpload];
  </code></pre>
  
  <p>and i get empty result.</p>