title: flask sqlalchemy column constraint for positive integer
body: <p>how can i define a column as a positive integer using flask sqlalchemy?</p>
  
  <p>i am hoping the answer would look something like this:</p>
  
  <pre><code>class City(db.Model):
      id = db.Column(db.Integer, primary_key=True)
      population = db.Column(db.Integer, positive=True)
      def __init__(self,population):
          self.population = population
  </code></pre>
  
  <p>however, this class definition will throw an error b/c sqlalchemy does not know about a 'positive' argument.</p>
  
  <p>i could raise an exception if an object is instantiated with a negative value for the population. but i don't know how to ensure that the population remains positive after an update.</p>
  
  <p>thanks for any help.</p>