class inherit_details(osv.osv):

_inherit="project.task"

_inherit="sale.order.line"

_columns = {

'description':fields.text('Description',size=10),

'debit': fields.boolean('Debit'),

'credit': fields.boolean('Credit'),

'salesDetail':fields.char('SalesDetails',size=20,required=True),

 'salestax':fields.char('SalesTax',size=10,required=True),

}

inherit_details()

------------------------------------------------xml file--------------------

<record id="inherit1_form_view" model="ir.ui.view">

<field name="name">project.task.form</field>

<field name="model">project.task</field>

<field name="type">form</field>

<field name="inherit_id" ref="project.view_task_form2"></field>

<field name="arch" type="xml">

<xpath expr="/form/sheet/group/group/field[@name='project_id']"

position="before">

<field name="description" />

</xpath>

<xpath expr="//field[@name='user_id']"

position="after">

<field name="debit" type="checkbox"/>

<field name="credit" type="checkbox"/>

</xpath>

</field>

</record>

<record id="saleinherit_form_view" model="ir.ui.view">

<field name="name">sale.order.form</field>

<field name="model">sale.order</field>

<field name="type">form</field>

<field name="inherit_id" ref="sale.view_order_form"></field>

<field name="arch" type="xml">

<xpath expr="/form/sheet/notebook/page[@string='Order Lines']/field[@name='order_line']/tree/field[@name='product_id']"

position="attributes">

<attribute name="string">ProductDetails</attribute>

</xpath>

<xpath expr="/form/sheet/notebook/page[@string='Order Lines']/field[@name='order_line']/tree/field[@name='price_subtotal']"

position="before">

<field name="salesDetail" />

</xpath>

<xpath expr="/form/sheet/notebook/page[@string='Order Lines']/field[@name='order_line']/tree/field[@name='tax_id']"

position="replace">

<field name="salestax" />

</xpath>

</field>

</record> 

-------------------------------------------------------------------Run----------------------------------

if  run above code, it produce error below

ERROR:

ParseError: "ValidateError

Field(s) `arch` failed against a constraint: Invalid view definition

Error details:

Field `credit` does not exist

Error context:

View `project.task.form`

[view_id: 1221, xml_id: Inheritance.inherit1_form_view, model: project.task, parent_id: 785]" while parsing /home/next/workspace/odoo-8/openerp/addons/Inheritance/views/inheritance.xml:5, near

<record id="inherit1_form_view" model="ir.ui.view">

<field name="name">project.task.form</field>

<field name="model">project.task</field>

<field name="type">form</field>

<field name="inherit_id" ref="project.view_task_form2"/>

<field name="arch" type="xml">

<data><xpath expr="/form/sheet/group/group/field[@name='project_id']" position="before">

<field name="description"/>

</xpath>

<xpath expr="//field[@name='user_id']" position="after">

<field name="debit" type="checkbox"/>

<field name="credit" type="checkbox"/>

</xpath>

</data></field>

</record>