computed field that does not compute and default that does not work (odoo-12) - odoo-12

Question

I have this field with a method in my model which aims to detect the changes in the many2one:
manytoone = fields.Many2one(comodel_name="product.template", string="Many to One",
required=False, )
cambio_realizado = fields.Boolean(string="Cambios en Many to One", default=False)
#api.onchange('manytoone')
def _detectar_cambio(self):
self.cambio_realizado = True
No works the default=False in Boolean Field. Always start in True.
And the other problem is that no works a computed field:
#api.multi
def computer_function(self):
self.computation = 243+234
computation = fields.Float(string="Result: ", compute=computer_function, readonly=True)
The result is zero... always in the view.
identation ok, and the model is inherited
My Class and my view with the tips:
class CustomModule(models.Model):
_inherit = 'sale.order'
name = fields.Char('campos relacionales, decoradores onchange y funcion computar')
manytoone = fields.Many2one(comodel_name="product.template", string="Many to One",
required=False, )
cambio_realizado = fields.Boolean(string="Cambios en Many to One")
#api.onchange('manytoone')
def _detectar_cambio(self):
self.cambio_realizado = True
onetomany = fields.One2many(comodel_name="sale.order", inverse_name="manytoone",
string="One to Many", required=False, )
manytomany = fields.Many2many(comodel_name="sale.order",
relation="sale_handler",
column1="order_id", column2="order_handler_id",
string="Many to Many", )
#api.depends('computation')
def computer_function(self):
for record in self:
record.computation = 300
computation = fields.Float(string="Result: ", compute=computer_function, readonly=True)
<odoo>
<!-- Inherit Form View to Modify it -->
<record id="custom_view_custom" model="ir.ui.view">
<field name="name">custom.view.custom</field>
<field name="model">sale.order</field>
<field name="inherit_id" ref="sale.view_order_form"/>
<field name="arch" type="xml">
<xpath expr="//sheet" position="inside">
<group>
<field name="manytoone"/>
<field name="onetomany"/>
<field name="manytomany"/>
</group>
<group>
<field name="computation"/>
<field name="cambio_realizado"/>
</group>
</xpath>
</field>
</record>
</odoo>

Answer 1

#api.one
def computer_function(self):
self.computation = 243+234
computation = fields.Float(string="Result: ", compute=computer_function, readonly=True)
use #api.one in your code

Answer 2

1- onchnage, when you try to create a new record, the onchange called automatically then cambio_realizado set to true
#api.onchange('manytoone')
def _detectar_cambio(self):
for o in self:
if o.manytoone:
o.cambio_realizado = True
2- in compute it's preferally to use api.depends('somefield'), but the code bellow may be work for you, use a loop and don't use readonly
#api.multi
def computer_function(self):
for o in self:
o.computation = 243+234
computation2 = fields.Float(string="Result: ", compute=computer_function)