title: Filter not working on hybrid property
body: <p>I am implementing SQLAlchemy ORM in a small blog app I am creating (as a learning exercise to Alchemy). 
  I have stumbled upon something I am not sure of - I think I know an approach, but it may be too long winded to be the "best" one. One table / object has a "title" column. I want to be able to create a slug type string from this. I looked at <a href="http://docs.sqlalchemy.org/en/rel_0_9/orm/mapper_config.html#using-a-hybrid/" rel="nofollow" title="hybrid properties">hybrid properties</a> and it seemed to do the trick. </p>
  
  <pre><code>class Post(Base):
      __tablename__ = 'posts'
      id = Column(Integer, primary_key=True)
      title = Column(String)
  
      @hybrid_property
      def slug(self):
          return self.title.replace(" ", "-").lower()
  
  
      def __repr__(self):
          return "&lt;Post(id='%s', title='%s', slug='%s')&gt;" % (
              self.id, self.title, self.slug)
  
  post = Post(title="Hello World")
  session.add(post)
  session.commit()
  </code></pre>
  
  <p>This works fine for a retrieval of the value:</p>
  
  <pre><code>&gt;&gt;&gt; p = session.query(Post).filter(Post.title=='Hello World')
  &gt;&gt;&gt; p 
  &gt;&gt;&gt; &lt;Post(id='1', title='Hello World', slug='hello-world')&gt;    
  </code></pre>
  
  <p>However when I try to use a filter on this property:</p>
  
  <pre><code>&gt;&gt;&gt; p = session.query(Post).filter(Post.slug=='hello-world')  
  </code></pre>
  
  <p>I get this error:</p>
  
  <pre><code>&gt;&gt;&gt; File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/attributes.py", line 270, in __ge
  tattr__
  key)
  AttributeError: Neither 'InstrumentedAttribute' object nor 'Comparator' object associated with
  Post.title has an attribute 'replace' 
  </code></pre>
  
  <p>Does this mean I should create a <a href="http://docs.sqlalchemy.org/en/rel_0_9/orm/internals.html#sqlalchemy.orm.interfaces.PropComparator" rel="nofollow">custom Comparator</a>? Seems a lot of work, for what would be a one line in most sql. Basically, is my whole approach flawed?  </p>