Hi,

Try refer following blog

https://www.cybrosys.com/blog/how-to-save-many2many-field-in-odoo-13-settings

Regards

Hello Frodo Baggins,

Suppose xyz named object is available and set M2M in res.config.settings object. 

class ResConfigSettings (models.TransientModel): 
       _inherit = 'res.config.settings' 

        xyz_id = fields.Many2one ('xyz') 
        xyz_ids = fields.Many2many ( 
                        'xyz', 
                        'xyz_res_config_settings_rel', 
'xyz table's field ',

 'xyz_id')

note: Many2many field can't see in database.

Regards,

Email:   odoo@aktivsoftware.com

Skype: kalpeshmaheshwari

   

res.config.settings is a transient model so we could not store anything in it.
you have to make m2m field either compute filed with inverse or related field.

https://github.com/odoo/odoo/blob/13.0/addons/crm/models/res_config_settings.py#L24