How to use many2many in OpenERP

Question

Help me to resolve this . My program is as follows I have declared many2many field in OpenERP if I choose a record to one customer it will not available to the other how its possible, The below program is mine. Help me.

from openerp.osv import orm, fields

class post_man(orm.Model):
    _name = "man"

    _columns = {
        'name':fields.char("Name", size=50, required=True),
        'description':fields.char("description", size=50),
        'branch_id':fields.many2one('branches', 'Branches'),
        'letter_lot':fields.many2many('letter', 'postman_letter_post', 'letter_id', 'pincode', 'Total Letters'),
#         'let_id':fields.many2one('branches','emp_id','Letter')
    }

    from openerp.osv import orm, fields

    class post_letter(orm.Model):
        _name = "letter"
        _rec_name="pin"
        _columns = {
            "name":fields.char("Sender Name",size=50, required=True),
            "address":fields.text("Destination Address"),
            "pin":fields.char("Postal code",size=25),
            "pincode":fields.many2one('branches','Destination'),
            "postman_id":fields.one2many('man','letter_lot','PMan')
        }

from openerp.osv import orm, fields

class post_man(orm.Model):
    _name = "man"

    _columns = {
        'name':fields.char("Name", size=50, required=True),
        'description':fields.char("description", size=50),
        'branch_id':fields.many2one('branches', 'Branches'),
        'letter_lot':fields.many2many('letter', 'postman_letter_post', 'letter_id', 'pincode', 'Total Letters'),
#         'let_id':fields.many2one('branches','emp_id','Letter')
    }

from openerp.osv import orm, fields

class post_letter(orm.Model):
    _name = "letter"
    _rec_name="pin"
    _columns = {
        "name":fields.char("Sender Name",size=50, required=True),
        "address":fields.text("Destination Address"),
        "pin":fields.char("Postal code",size=25),
        "pincode":fields.many2one('branches','Destination'),
        "postman_id":fields.one2many('man','letter_lot','PMan')
    }