Hi Wombat, this should find the qty sold for a product. Also you have to add it to the view. Again I think you should also consider uom. Another thing is there is already a field in the product template form to show the sale qty. The field is called 'sales_count'. So this funtion or a new field should be redundent. 

from odoo import api, models, fields

class ProductsSold(models.Model):
    _inherit = "product.product" # database table name.

    def _compute_avail(self):
        records_sum = sum(self.env["sale.order.line"].search([('product_id', '=', self.id), ('state', '=', 'sale')]).mapped('product_uom_qty'))
        self.sold = records_sum    sold = fields.Float(compute='_compute_avail', string="Avail")
instead this below should do want you want to achive. and no need to create any field or method. in words you need to inherit the tree view and just add the sales_count field. **Code section in comment does not let me include closing tags**

    product_template_inherit_treefield>
    product.templatefield>
    
    

        
            
        xpath>

    field>
record>