title: How do I declare a base model class in Flask-SQLAlchemy?
body: <p>I would like to declare a base class that all other schema objects
  can inherit from, for example: </p>
  
  <pre><code>class Base(db.Model):
      created_on = db.Column(db.DateTime, default=db.func.now())
      updated_on = db.Column(db.DateTime, default=db.func.now(), onupdate=db.func.now())
  </code></pre>
  
  <p>Then all other schema objects can inherit from it and not have to repeat
  the declaration of the two columns. </p>
  
  <p>How would I do this in Flask-SQLAlchemy? </p>
  
  <pre><code>from flask.ext.sqlalchemy import SQLAlchemy
  db = SQLAlchemy(app)
  
  class User(db.Model):
      __tablename__ = 'users'
      id = db.Column(db.Integer, primary_key = True)
      email = db.Column(db.String(255), unique = True)
  </code></pre>