title: How is a unique constraint across three columns defined?
body: <p>The following <code>EventInvitation</code> model is a simple invitation for one event, sent from a user to another user. I would like to ensure that the invitations are unique across three columns: to_user, from_user and event.</p>
  
  <pre><code>class EventInvitation(db.Model):
      __tablename__ = 'event_invitations'
  
      id = db.Column(db.Integer, primary_key = True)
  
      event_id = db.Column(db.Integer, db.ForeignKey('events.id'))
      event = db.relationship('Event',  foreign_keys=[event_id])
      created = db.Column(db.DateTime(), default=datetime.now)
      updated = db.Column(db.DateTime(), default=datetime.now,onupdate=datetime.now)
  
      from_id = db.Column(db.Integer, db.ForeignKey('users.id'))
      from_user = db.relationship('User',  foreign_keys=[from_id])
  
      to_id = db.Column(db.Integer, db.ForeignKey('users.id'))
      to_user = db.relationship('User',  foreign_keys=[to_id])
  
      cstrt = db.UniqueConstraint('event_id', 'from_id','to_id', name='uix_1')
  </code></pre>
  
  <p>I tried with this <code>cstrt</code> column but it doesn't work.  The constraint should work on SQLite, as well as on MySQL in production.  How can I define this unique constraint?</p>