title: Flask-Admin: Add custom property to column_list
body: <p>I need to add a custom property of a SQLAlchemy model class to my Flask-Admin view. It does some calculation which I want to display in my column list.  </p>
  
  <pre><code>class Withdrawal(db.Model):   
      __tablename__ = 'withdrawals'
      id = db.Column(db.Integer, primary_key=True)
      fabric_id = db.Column(db.Integer, db.ForeignKey('fabrics.id'))
      fabric  = db.relationship('Fabric', backref='withdrawals')
      length = db.Column(db.Float)
  
  class Fabric(db.Model):
      __tablename__ = 'fabrics'
      name = db.Column(db.String(256))
      id = db.Column(db.Integer, primary_key=True)
      org_length = db.Column(db.Float)
  
      @property
      def length(self):
          return self.org_length - sum(x.length for x in self.withdrawals)
  </code></pre>
  
  <p>I added the <code>Fabric.length</code> property to my views <code>column_list</code> but won't get to see a value in the new column.</p>
  
  <pre><code>class FabricModelView(ModelView):   
      column_list = ('org_length', Fabric.length)
  </code></pre>
  
  <p>The result should be a column with the calculated length but actually nothing is shown in the added column.</p>
  
  <p><a href="https://i.stack.imgur.com/5L4vS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5L4vS.png" alt="Screenshot"></a></p>
  
  <p>So my question is: Is there a way to add a custom property to the column list in a Flask-Admin view?</p>