code: def add(cls, model, commit=True):
          
          if not isinstance(model, cls):
              raise ValueError('%s is not of type %s' % (model, cls))
          cls.session.add(model)
          try:
              if commit:
                  cls.session.commit()
              return model
          except:
              cls.session.rollback()
              raise
docs: Adds a model instance to session and commits the
          transaction.
  
          Args:
  
              model: The instance to add.
  
          Examples:
  
              >>> customer = Customer.new(name="hari", email="hari@gmail.com")
  
              >>> Customer.add(customer)
              hari@gmail.com