My code: Code below makes no difference in field.

backend_id = fields.Many2one(comodel_name='qb.backend',
                                      string='Quick Backend', store=True,
                                      readonly=False, required=False, default=lambda self:
                                      self.env['qb.backend'].search([]).id
                                      )

I want to set default value of the field in existing records. As I am able to set that field with default value at the time of creating new record with the help of default_get function, but existing records need to be filled manually or by selecting the value.

Example:

_name='qb.backend'

name = fields.char()

 When this field is saved, I want the value of this field to be assigned to the many2one related field in other models.

Backend field exist in every module so the existing database or existing records having this field should be auto filled on save of above field called name.

Backend field should be auto filled by the value of above field.

backend = fields.Many2one('comodel='qb.backend')