I did it this way:
first I created a new field type Many2many:
objectA_ids = fields.Many2many('res.partner')
then a function that gets the value of the old Many2one field:
products = self.env['module.name'].search(
['&', ('objectA_id', '!=', False), ('objectA_ids', '=', False)])
for r in products:
if not r.objectA_ids and r.objectA_id:
r.objectA_ids = [(4, r.objectA_id.id)]
r.objectA_ids = False
and in the last step, i did make an XML call to the function when the module it's installing:
<function model="module.name" name="_fill_objects"/>
after installing the module the new field has the values of the old field, there's no way for change the field type without lost data, so we have to create a new field, copy data from the old field, and make the old field invisible on view.
<record id="view_module_name_adjustment" model="ir.ui.view">
<field name="inherit_id" ref="module_view_external_id"/>
<field name="arch" type="xml">
<field name="objectA_id" position="after">
<field name="objectA_ids" widget="many2many_tags"/>
<field name="objectA_id" position="attributes">
Why would you want to change from a many2one to a many2many though?You're most likely better off to create a one2many that links to a many2one - it'll behave the same as a many2many but it has benefits. On a many2many you cannot filter by default and if you export the data you won't get everything out automatically. With a one2many you can. Just a tip. :-)
@Yenthe Your approach it's helpful, I've not even though, so, can I take your answer as If I can change the field from Many2one to One2many or Many2many and the data persist?
I can't use the One2many field because 1 a.object can have N b.object and 1 b.object can be in N a.object, so I should use the Many2many field, but I need to know if the information persists after changing the field type from one2Many to (Many2many or One2many).