2 fields in tree view - odoo

Question

In the product category, I added a product_ids field and I want to display it with a tree view showing name and default_code of products. For some reason I get the error "Field default_code does not exist"
<record id="view_product_category_qty_discount" model="ir.ui.view">
<field name="name">product.category.inherit.qty.discount.Config Hetlita</field>
<field name="model">product.category</field>
<field name="type">form</field>
<field name="inherit_id" ref="product.product_category_form_view" />
<field name="arch" type="xml">
<form position="inside">
<group col="2" colspan="2">
<separator string="Quantity for discount" colspan="2"/>
<field name="qty_for_discount" />
</group>
<group>
<field name="product_ids" widget="many2many_tags"/>
<tree>
<field name="name"/>
<field name="default_code"/>
</tree>
</group>
</form>
</field>
</record>
class ProductCategory(models.Model):
_inherit = 'product.category'
qty_for_discount = fields.Float(string='Qty For Discount')
product_ids = fields.Many2many(
'product.template', string='Products')

Answer 1

That's because there is no default_code on model product.template but instead on its variants with model product.product. I would change the field on product.category to a One2Many on product.product:
product_ids = fields.One2many(
comodel_name='product.product',
inverse_name='categ_id',
string='Products')
And there is a mistake in your xml:
<group>
<field name="product_ids">
<tree>
<field name="name"/>
<field name="default_code"/>
</tree>
</field>
</group>