title: backref class attribute
body: <p>How to initialize backrefs of mappers without some queries through a session? 
  For example, I have two models, named "Client" and "Subject" in follow code:</p>
  
  <pre><code>Base = declarative_base()
  
  class Client(Base):
      __tablename__ = "clients"
  
      id = Column(Integer, primary_key=True)
      created = Column(DateTime, default=datetime.datetime.now)
      name = Column(String)
  
      subjects = relationship("Subject",  cascade="all,delete",
          backref=backref("client"))
  
  
  class Subject(Base):
      __tablename__ = "subjects"
  
      id = Column(Integer, primary_key=True)
      client_id = Column(Integer, ForeignKey(Client.id, ondelete='CASCADE'))
  </code></pre>
  
  <p>Then, somewhere in my code, I want to get the backref <code>client</code> of the class <code>Subject</code> like this, but that raises an exception:</p>
  
  <pre><code>&gt;&gt;&gt; Subject.client
  AttributeError: type object 'Subject' has no attribute 'client'
  </code></pre>
  
  <p>After a query to <code>Client</code> like:</p>
  
  <pre><code>&gt;&gt;&gt; session.query(Client).first()
  &gt;&gt;&gt; Subject.client
  &lt;sqlalchemy.orm.attributes.InstrumentedAttribute at 0x43ca1d0&gt;
  </code></pre>
  
  <p>Attribute <code>client</code> was created after a query to the related model(mapper).<br>
  I don't want to make such "warming" queries!</p>