create one xml file  for sequencing---

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data noupdate="1">

        <!-- Sequences for loan_id -->
        <record id="seq__loan_code_inh1" model="ir.sequence.type">
            <field name="name">Loan ID</field>
            <field name="code">lending.info</field>
        </record>

        <record id="seq_cloan_code_inh2" model="ir.sequence">
            <field name="name">Loan ID</field>
            <field name="code">lending.info</field>
            <field name="prefix">LN</field>
            <field name="padding">2</field>
            
        </record>

    </data>
</openerp>

add  this xml file into your openerp.py file -------

{
'name':'Money Lend ',
'description':"""Module for managing small scale customers""",
'version':'0.0',
'author':'Libu Koshy m',
'depends':['base'],
'update_xml' : ['loan_view.xml',
                'loanseq_id.xml'],
'installable':True,
'category':'General',
'description':'Financial module Demo',
'website':'',
'demo':[]
}
----------------------------------------

override the create method and add into your custom class

class lending_info(osv.osv):
    
    _name='lending.info'
  
    
    _columns={
          
              'loan_id':fields.char('ID',readonly=True),

              }

def create(self, cr, uid, vals, context=None):
        vals['loan_id'] = self.pool.get('ir.sequence').get(cr, uid,'lending.info')
        return super(lending_info, self).create(cr, uid, vals, context=context)

 _defaults={
                'loan_id': lambda obj, cr, uid, context: '/',

                }