I'm using v10 and I'm trying to add few fields to 'res.partner' and then add the same fields to other forms.

Here are my .py and .xml:

class add_vendor_ship_carrier(models.Model):
    _inherit = 'res.partner'
    ship_carrier = fields.Char('Ship Carrier')    
    ship_carrier_account = fields.Char('Ship Carrier Account')

class add_to_sales_order(models.Model):
    _inherit = 'sale.order'    
    ship_carrier = fields.Char(related='partner_id.ship_carrier')    
    ship_carrier_account = fields.Char(related='partner_id.ship_carrier_account')

class add_to_purchase_order(models.Model):
    _inherit = 'purchase.order'    
    ship_carrier = fields.Char(related='partner_id.ship_carrier')    
    ship_carrier_account = fields.Char(related='partner_id.ship_carrier_account')

<record id="ship_carrier_supplier_form" model="ir.ui.view"> 
   <field name="name">vendor.carrier.inherit</field> 
   <field name="model">res.partner</field> 
   <field name="inherit_id" ref="base.view_partner_form"/> 
   <field name="priority">36</field> 
   <field name="arch" type="xml"> 
       <field name="property_stock_supplier" position="after">   
           <field name="ship_carrier"/>                    
           <field name="ship_carrier_account"/>   
       </field> 
   </field> 
</record>

<record id="ship_carrier_sale_order_form" model="ir.ui.view"> 
    <field name="name">customer.carrier.sale.order.inherit</field> 
    <field name="model">sale.order</field> 
    <field name="inherit_id" ref="sale.view_order_form"/> 
    <field name="priority">36</field> 
    <field name="arch" type="xml"> 
        <field name="fiscal_position_id" position="after"> 
            <field name="ship_carrier"/> 
            <field name="ship_carrier_account"/> 
        </field> 
    </field> 
</record>

<record id="supplier_carrier_purchase_order_form" model="ir.ui.view"> 
   <field name="name">supplier.carrier.purchase.order.inherit</field> 
   <field name="model">purchase.order</field> 
   <field name="inherit_id" ref="purchase.purchase_order_form"/> 
   <field name="priority">36</field>            
   <field name="arch" type="xml"> 
       <field name="date_planned" position="after"> 
           <field name="ship_carrier"/> 
           <field name="ship_carrier_account"/> 
       </field> 
   </field> 
</record>

It added my 2 custom fields to 'res.partner' and to 'sale.order' forms, but not to 'purchase.order' form.

I'm getting the following error:

ValueError: Can't validate view:
Field `ship_carrier` does not exist

I'm trying to use related fields to add them to multiple forms, but it doesn't work for purchase order for some reason. 

What am I doing wrong and what is the easiest way to add a field to multiple forms?

Thanks for help in advance.