在早前的odoo13或更早版本，一直有 select_add 方法，用于给 selection 增加值。但问题是不可以定义在什么位置，这时我们只能重新定义了。 但是由于 odoo 继承的特性，经常我们不知道哪个别的模块也新增加了一个 select。 在最新的版本中，增加了处理方法，来解决这种问题。
例：我们要在 销售订单中， 状态增加 处理。 销售报价发送前须【已审批】。 销售开票前须【已复核】，
以下是我们完整的销售审批模块，在 odoo 市场上可以找到， app_sale_approval
state = fields.Selection(selection_add=[
('to_approve', 'To Approve'),
('sent',),
('to_check', 'To Check'),
('done',),
])
同时，还对 selection 增加了 ondelete方法，配合selection_add，用于当模块卸载时处理数据。
:param ondelete: provides a fallback mechanism for any overridden
field with a selection_add. It is a dict that maps every option
from the selection_add to a fallback action.
This fallback action will be applied to all records whose
selection_add option maps to it.
The actions can be any of the following:
- 'set null' -- the default, all records with this option
will have their selection value set to False.
- 'cascade' -- all records with this option will be
deleted along with the option itself.
- 'set default' -- all records with this option will be
set to the default of the field definition
- <callable> -- a callable whose first and only argument will be
the set of records containing the specified Selection option,
for custom processing
