title: SQLAlchemy: 'Model' object is not callable
body: <p>I'm using Flask framework with sqlalchemy and I can't query some related table of a result.</p>
  
  <p>Users > have many > orders</p>
  
  <p>So I have the order and I want to query the user that order belongs to.</p>
  
  <pre><code>order = Order.query.get( id )
  return order.user()
  </code></pre>
  
  <p>When I call this I get that the 'User' object is not callable. Here's how my relation is defined:</p>
  
  <pre><code>#   ----------------------------------------------------------------------------------
  #   MODELS
  #   ----------------------------------------------------------------------------------
  class User( object ):
      __tablename__ = 'user'
  
      query = db_session.query_property()
  
  class Order( object ):
      __tablename__ = 'order'
  
      query = db_session.query_property()
  
  #   ----------------------------------------------------------------------------------
  #   MODELS SCHEMA
  #   ----------------------------------------------------------------------------------
  users_table = Table( 'user', metadata,
      Column( 'id', Integer, primary_key = True ),
      Column( 'username', String( 50 ), unique = True ),
      Column( 'email', String( 50 ), unique = True ),
      Column( 'password', String( 32 ), nullable = False ),
      ...
      mysql_engine = 'InnoDB',
      mysql_charset = 'utf8'
   )
  
  orders_table = Table( 'order', metadata,
      Column( 'id', Integer, primary_key = True ),
      Column( 'user_id', Integer, ForeignKey( 'user.id' ) ),
      Column( 'transaction_id', String( 32 ), unique = True ),
      ...
      mysql_engine = 'InnoDB',
      mysql_charset = 'utf8'
   )
  
  #   ----------------------------------------------------------------------------------
  #   MAPPERS
  #   ----------------------------------------------------------------------------------
  mapper( User, users_table, properties = {
      #   has many
      'orders' : relationship( Order )
  } )
  
  mapper( Order, orders_table, properties = {
      #   belongs to
      'user': relationship( User )
  } )
  </code></pre>
  
  <p>Any idea why this error?</p>