title: How to add some extra data to a symfony 2 form
body: <p>I have a form for my entity called <code>Book</code> and I have a type to display a form in my view. In this type I have some fields that are mapped to properties in my entity. </p>
  
  <p>Now I want to add another field which is not mapped in my entity and supply some initial data for that field during form creation.</p>
  
  <p><strong>My Type looks like this</strong></p>
  
  <pre><code>// BookBundle\Type\Book
  public function buildForm(FormBuilderInterface $builder, array $options = null)
  {
      $builder-&gt;add('title');
      $builder-&gt;add('another_field', null, array(
          'mapped' =&gt; false
      ));
  }
  </code></pre>
  
  <p><strong>The form is created like this</strong></p>
  
  <pre><code>$book = $repository-&gt;find(1);
  $form = $this-&gt;createForm(new BookType(), $book);
  </code></pre>
  
  <p>How can I supply some initial data now during form creation? Or how do I have to change that creation of the form to add initial data to the <code>another_field</code> field?</p>