title: Django Haystack: error trying to build solr schema
body: <p>I'm getting the following error when attempting to run <code>./manage.py build_solr_schema</code></p>
  
  <pre><code>NotImplementedError: Subclasses must provide a way to build their schema.
  </code></pre>
  
  <p>Here are what my two search indexes look like:</p>
  
  <pre><code>class BookSearchIndex (SearchIndex):
      text = CharField(document=True, use_template=True)
      title = CharField(model_attr='title_web', boost=1.125)
  
      def index_queryset(self):
          return Book.objects.active().filter(publish_level='published')
  
  site.register(Book, BookSearchIndex)
  
  
  class AuthorSearchIndex (SearchIndex):
      text = CharField(document=True, use_template=True)
      name = CharField(model_attr='name_display', boost=1.5)
  
      def index_queryset(self):
          return Author.objects.approved()
  
      def prepare(self, obj):
          data = super(AuthorSearchIndex, self).prepare(obj)
          data['boost'] = 1.5
          return data
  
  site.register(Author, AuthorSearchIndex)
  </code></pre>
  
  <p>I'm running this locally and using the simple backend.  I was able to run <code>build_solr_schema</code> after creating the authors index.  But then when I setup the books index and tried to run it again I got the error mentioned.</p>
  
  <p>Django 1.4.2,
  Haystack 1.2.7</p>
  
  <p>Any ideas?</p>