title: SQLAlchemy - Trying Eager loading.. Attribute Error
body: <p>I access a a postgres table using SQLAlchemy. I want a query to have eagerloading.</p>
  
  <pre><code>from sqlalchemy.orm import sessionmaker, scoped_session, eagerload
  from settings import DATABASE_USER, DATABASE_PASSWORD, DATABASE_HOST, DATABASE_PORT, DATABASE_NAME
  from sqlalchemy import create_engine
  from sqlalchemy import Table, Column, Integer, String, Boolean, MetaData, ForeignKey
  from sqlalchemy.orm import mapper
  from sqlalchemy.ext.declarative import declarative_base
  
  def create_session():
    engine = create_engine('postgres://%s:%s@%s:%s/%s' % (DATABASE_USER, DATABASE_PASSWORD, DATABASE_HOST, DATABASE_PORT, DATABASE_NAME), echo=True)
    Session = scoped_session(sessionmaker(bind=engine))
    return Session()
  
  Base = declarative_base()
  class Zipcode(Base):
    __tablename__ = 'zipcode'
    zipcode = Column(String(6), primary_key = True, nullable=False)
    city = Column(String(30), nullable=False)
    state = Column(String(30), nullable=False)
  
  session = create_session()
  query = session.query(Zipcode).options(eagerload('zipcode')).filter(Zipcode.state.in_(['NH', 'ME']))
  #query = session.query(Zipcode.zipcode).filter(Zipcode.state.in_(['NH', 'ME']))
  print query.count()
  </code></pre>
  
  <p>This fails with
  AttributeError: 'ColumnProperty' object has no attribute 'mapper'</p>
  
  <p>One without eagerloading returns the records correctly.</p>
  
  <p>I am new to SQLAlchemy. I am not sure what the problem is. Any pointers?</p>