Hi,
This is because, you defined domain or attributes for one2many field inside the tree view
INCORRECT :
<page string="Purchase Request">
<field name="order_line"> ----------- (one2many field)
    <tree string="Purchase Req Order Lines" editable="bottom">
          <field name="part_name"  attrs="{'readonly':[('state','not in',('draft',))]}"/>
            <field name="price_unit"   attrs="{'readonly':[('state','not in',('draft',))]}"/>
            <field name="sub_total" attrs="{'readonly':[('state','not in',('draft',))]}" />
    </tree>
</field>
</page>
Correct :
<page string="Purchase Request">
<field name="order_line" attrs="{'readonly':[('state','not in',('draft',))]}"> ----------- (one2many field)
        <tree string="Purchase Req Order Lines" editable="bottom">
                  <field name="part_name" />
                   <field name="price_unit"/>
                    <field name="sub_total" />
        </tree>
   </field>
</page>
- state field is present only in main class. Hope this may solve your problem