title: Rails 3 - Calculated column
body: <p>I want to create a calculated field that multiply column1 and column2 on rails 3. </p>
  
  <p>Like this : </p>
  
  <pre><code>totalpoint = column1 * column2
  </code></pre>
  
  <p>Where is I have to place the code? in the model? How do I write it?</p>
  
  <p>How do I call it from my view?</p>