title: SQLAlchemy show me that "AttributeError: type object 'User' has no attribute 'columns'"
body: <p>I am building a small project use python+Flask+SQLAlchemy, I make a model file following:</p>
  
  <pre><code>################# start of models.py #####################
  from sqlalchemy import Column, Integer, String, Sequence, Date, DateTime, ForeignKey
  from sqlalchemy.orm import relationship, backref
  from dk.database import Base
  import datetime
  
  class User(Base):
      __tablename__ = 'users'
      id = Column(Integer, Sequence('seq_user_id'), primary_key=True)
      name = Column(String(50), unique=True, index = True, nullable = False)
      email = Column(String(120), unique=True, index = True, nullable = False)
      password = Column(String(128), nullable = False)
  
      def __init__(self, name, email, password):
          self.name = name
          self.email = email
          self.password = password
  
      def __repr__(self):
          return '&lt;User %r&gt;' % (self.name)
  
  class Session(Base):
      __tablename__ = 'session'
      id = Column(String(128), primary_key = True, nullable = False)
      user_name = Column(String(30), nullable = False)
      user_id = Column(Integer, ForeignKey('users.id'))
      user = relationship('User', backref=backref('session', lazy='dynamic'))
  
      def __repr__(self):
          return '&lt;Session %r&gt;' % (self.id)
  ################# end of models.py #####################
  </code></pre>
  
  <p>and I build a initial file following:</p>
  
  <pre><code>################# start of __init__.py #################
  from flask import Flask
  from flask.ext.sqlalchemy import SQLAlchemy
  
  app = Flask(__name__)
  app.config.from_object('config') #load database config information
  db = SQLAlchemy(app)
  ################# end of __init__.py #################
  </code></pre>
  
  <p>when I run the "init_db()" in script, tables built to database successful.
  but when I want to see the SQL script then I run the "print CreateTable(User)" in script, the system show follwing errors:</p>
  
  <pre><code>  File "/home/jacky/flaskcode/venv/lib/python2.6/site-packages/sqlalchemy/schema.py", line 3361, in __init__
      for column in element.columns
  AttributeError: type object 'User' has no attribute 'columns'
  </code></pre>
  
  <p>I have no idea how to solve this problem!</p>