I want to, when user selected (through Many2one field from product_category) a category, I need to find its related parent_id for Brands to add a domain filter on another Many2one field to have related child entries of these Brands. As I am beginner I failed to how to do this.

Suppose Many2one field category_id here, user will select a category from first three entries, now we know that user wants to add an Electronics related item then system should populate Many2one field brand_id on Electronics related Brands (except All / Electronics / Brands) to select from below last 2 entries here in All / Electronics / Brands.

All / Electronics / Personal Computers
All / Electronics / Personal Computers / Laptop
All / Electronics / Personal Computers / Desktop
... 
...
All / Electronics / Brands
All / Electronics / Brands / HP
All / Electronics / Brands / IBM

There are more brands in this model (table data) like:

All / Foods / Brands
All / Foods / Brands / Hershey
All / Foods / Brands / Heinz
All / Foods / Brands / Oreo
...
All / Services / Brands
All / Services / Brands / IBM Garages
All / Services / Brands / Telos MNS
All / Services / Brands / Relia Quest
...

My model class is here... asking for field brand_id to filter with related Brands except the first entry of Brands in the selected category.

class OrderItems(models.Model):
    _name = 'tests.orderitems'
    _description = "Tests Order Items"
    store_id = fields.Many2one('tests.stores', string="Store", ondelete='cascade')
    order_id = fields.Many2one('tests.testsorders')
    categry_id = fields.Many2one('product.category', string="Category",
                                 domain="[['complete_name', 'not like', '%Brands%']]")
    items_id = fields.Many2one('tests.storeitems', string="Item",
                               domain="[['categs_id', '=', categry_id]]")
    brand_id = fields.Many2one('product.category', string="Brand",
                               domain="[['complete_name', 'like', '%Brands%']]")
    unit_id = fields.Many2one('tests.units', string="Unit")
    quantity = fields.Integer(string="Quantity", required=True, default=1)
    rates = fields.Float(string="Rate", default=0)
    size = fields.Char(string="Size")

please help...