I have 2 custom module

1- Setup Customer Site module 

2- Site link Module is generating tree of site module

when i inherit generated tree into sale order it gives error of site_a field not found iam sharing the code 

class sites_link(models.Model):
    _name = 'sites.link'
    _inherit = ['customer.sites', 'res.partner']
    link_ids = fields.One2many('link.link', 'link_id', string='link')

class link_link(models.Model):
           _name = 'link.link'
           link_id = fields.Many2one('sites.link')
           site_a = fields.Many2one('customer.sites','Site A')
           site_b = fields.Many2one('customer.sites','Site B')
           linkname = fields.Char('Link Name')

<odoo>
<data>
 <act_window id="sites_link_action" name="sites_link tree" res_model="sites.link" />
    <menuitem id="sites_link_menu" name="sites link" action="sites_link_action" parent="" sequence="5" />
      <record id="view_order_form_custom" model="ir.ui.view">
            <field name="name">sale.order.form</field>
            <field name="model">sale.order</field>
         <field name="inherit_id" ref="sale.view_order_form"/>
            <field name="arch" type="xml">
            <div name="lnk" position="before">
                     <field name="link_ids" >
                                   <tree editable="bottom">
                              <field name="site_a"/>
                              <field name="site_b"/>
                               <field name="linkname"/>
                           </tree>
                     </field>
            </div>
         </field>
   </record>
</data>
</odoo>