title: Objective-C: Use category method when the class is known at runtime
body: <p>I'm using MagicalRecord and would like to be able to do something like</p>
  
  <pre><code>Car *newCar = [Car importFromObject:obj]; // where obj is a JSON object
  </code></pre>
  
  <p>but I would like to do it without prior knowledge of the class, having only the @"Car" string or Car as a Class object at runtime.</p>
  
  <p>I know how to create a new car</p>
  
  <pre><code>id obj = [[NSClassFromString(@"Car") alloc] init];
  </code></pre>
  
  <p>but how should I call the importFromObject: method ?</p>