I Am Creating Workflow for Approve Account button in odoo 10 i tried a lot but is not working can anyone tell me whats wrong with this code

Here is my code for creating Transition and activity

<record id="wkf_model_setting" model="workflow">           

    <field name="name">Name</field>          

     <field name="osv">model</field>           

    <field name="on_create">True</field>       

</record>

<!--Activity -->                

<record id="act_draft" model="workflow.activity">           

    <field name="wkf_id" ref="wkf_model_setting"/>           

    <field name="flow_start">True</field>           

    <field name="name">draft</field>           

    <field name="kind">function</field>           

    <field name="action">write({'state':'draft'})</field>       

</record>              

<record id="act_approved" model="workflow.activity">           

    <field name="name">approval</field>           

    <field name="wkf_id" ref="wkf_model_setting"/>           

    <field name="kind">function</field>           

    <field name="action">do_approval()</field>       

</record>

<record id="trans_awaiting_approved" model="workflow.transition">          

     <field name="act_from" ref="act_draft"/>           

    <field name="act_to" ref="act_approved"/>           

    <field name="signal">button_approval</field>           

    <field name="group_id" ref="PE_ADMIN"/>      

</record>

and defination for do_approval is like this

@api.multi   

def do_approval(self):
        #TODO: Check if user has rights       

         self.state ='approved'