Hi,

To achieve your goal you need to define one method in your class where you add that new field. And set data into existing records. Please have a look on the below code.

class calendar_event(...)
    _inherit = "event.calendar"
    category_id = Fields.many2one(....)

    def _auto_init(self, cr, context=None):
        cr.execute("update event_calendar set category_id = 1 where category_id is null")

I hope it will resolve your issue.

Thanks.

Old question but still interresting for newer versions. In 12.0 for example,  _auto_init() and init() are fine for indexes but for NOT NULL constrains, by the time those methods are executed, it's too late and the SQL constraints have already been applied and caused a NOT NULL constraint error due to existing rows with null data. It seems the way to handle this properly is by overriding the _init_column() method, check for the column name and do the SQL UPDATE statements in there:

    @api.model_cr_context
    def _init_column(self, column_name):
        # Set a value on existing data for required fields
        if column_name == 'real_check_in':
            self.env.cr.execute("UPDATE hr_attendance SET real_check_in = check_in WHERE real_check_in IS NULL")
        elif column_name == 'real_check_out':
            self.env.cr.execute("UPDATE hr_attendance SET real_check_out = check_out WHERE real_check_out IS NULL")
        super()._init_column(column_name)