How to prevent duplicate values in a table field

One of the ways to do it is to write the on_apply event handler.

In the example below, the delta parameter is a dataset that contains the changes that will be stored in the users table.

We go through the records of changes and if the record was not deleted or the login field didn’t change we look for a record in the table with the same login and if it exists raise the exception. If the user is editing the record on the client using an edit form he won’t be able to save it and will see the corresponding alert message.

def on_apply(item, delta, params, connection):
    for d in delta:
        if not (d.rec_deleted() or d.rec_modified() and d.login.value == d.login.old_value):
            users = d.task.users.copy(handlers=False)
            if users.rec_count:
                raise Exception('There is a user with this login - %s' % d.login.value)