class BuildingDefinition(models.Model):

    _name = 'building.definition'

    name = fields.Char(required=True, string='Building Name')

    main_building = fields.Many2one('building.definition')

class BuildingUnits(models.Model):

    _name = 'building.units'

    name = fields.Char()

    unit = fields.Many2one('building.definition')

@api.multi

    def unit_check(self):

        for rec in self:

            domain = [('main_building', '=', True)]

            only_main = self.env['building.definition'].search([]).mapped('main_building').mapped('name')

            all_build = self.env['building.definition'].search([]).mapped('name')

            res = list(set(all_build) - set(only_main))

            self.unit = [(6, 0, res.ids)]

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every building usually have a main building and units

in field unit i need buildings(at BuildingDefinition module with 'name') whcih are not main buildings ( at BuildingDefinition module with 'main_building)

tryed also domains in XML but still not working

kindely how to solve ?