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In Odoo, it's common to use One2many fields with inline "Add a line" functionality. A frequently requested feature is to auto-increment a numeric field, such as a sequence number, every time the user adds a new line.

This guide demonstrates how to auto-increment a field (e.g., sequence) in a One2many line when clicking “Add a line” in the Odoo form view.

Use Case

You have a model indoor.dashboard.device with a field called sequence. You want this field to increase by 1 each time a new line is added in the One2many list.

Parent Model

class IndoorDashboard(models.Model):
    _name = 'indoor.dashboard'
    _description = 'Indoor Dashboard Management'

    name = fields.Char(string='Name', required=True, copy=False, readonly=True,
                       default=lambda self: self.env['ir.sequence'].next_by_code('indoor.dashboard.sequence') or 'New')

    title = fields.Char(string='Title')
    item_date = fields.Datetime(string='Date', default=fields.Datetime.now) 
    author = fields.Char(string='Author')

    location = fields.Char(string='Location')
    remark_header = fields.Text(string='Remark') 

    state = fields.Selection([
        ('draft', 'Draft'),
        ('confirmed', 'Confirmed'),
        ('done', 'Done'),
    ], string='Status', default='draft', readonly=True, tracking=True) 

    device_ids = fields.One2many('indoor.dashboard.device', 'dashboard_id', string='Devices')

Child Model

class IndoorDashboardDeviceLine(models.Model):
    _name = 'indoor.dashboard.device'
    _description = 'Indoor Dashboard Device'

    dashboard_id = fields.Many2one('indoor.dashboard', string='Dashboard', required=True, ondelete='cascade')
    sequence = fields.Integer(string='No.') 

    name = fields.Char("Device")
    code = fields.Char("Node#")

    device_type = fields.Selection([('temperature', 'Temperature'), ('humidity', 'Humidity')], default='temperature', string="Type") 

    url = fields.Char("Url")
    remark = fields.Char("Remark")

    @api.onchange('dashboard_id')
    def _onchange_dashboard_id_sequence(self):
        if self.dashboard_id:  # When Click Add a Line on Form View
            existing_lines = self.dashboard_id.device_ids.filtered(lambda l: l != self)
            max_sequence = max(existing_lines.mapped('sequence') or [0])
            self.sequence = max_sequence + 1

Result

The solution is to use @api.onchange('dashboard_id') in the child model.

When you click Add a Line, the No. column (sequence) will automatically increment based on the existing lines.