How is a unique constraint across three columns defined?

Question

The following EventInvitation model is a simple invitation for one event, sent from a user to another user. I would like to ensure that the invitations are uniq

Answer 1

You need to add the constraint to the table, not the model. To do this using declarative:

class EventInvitation(db.Model):
    # ...
    __table_args__ = (
        db.UniqueConstraint(event_id, from_id, to_id),
    )

If the table has already been created in the database, you'll need to drop the table and run db.create_all() again, or use Alembic to alter the existing table with a migration.