While using these codes I am not able to save the data after clicking button on loadingslip. Is there any way to do that?
ORM code:
from odoo import models, fields, api

class sale_customer(models.Model):
    _inherit='res.partner'
    pan_number=fields.Char(string='PAN Number')
    vat_number = fields.Char(string='VAT Number')
    bank_name = fields.Char(string='Bank Name')
    acc_number = fields.Char(string='Account Number')
    # p = fields.Char(string='Verification Status')
    contrat_start=fields.Date(string='Contract start Date')
    contract_status=fields.Selection(
    [('', ''), ('yes', 'Yes'),('no', 'No')],
    default='', string='Contract Status')
    key_person=fields.Char(string='Key Person')

    mobile_no=fields.Char(string='Mobile Number')

   
class sale_code(models.Model):
    _inherit='sale.order'
    add=fields.Char(string='Address')

    @api.multi
    def open_second_class(self):
        ac = self.env['ir.model.data'].xmlid_to_res_id('agni_loading_dharma_bharpai.loadingslip_form_12', raise_if_not_found=True)
        tbl1 = False
        for o in self:
            tbl1 = o.id
        result = {
            'name': '2nd class',
            'view_type': 'form',
            'res_model': 'loadingslip',
            'view_id': ac,
            'context': {'default_id_so': tbl1},
            'type': 'ir.actions.act_window',
            'view_mode': 'form'
        }
        return result

    @api.multi
    def open_third_class(self):
        ac = self.env['ir.model.data'].xmlid_to_res_id('agni_loading_dharma_bharpai.dharmakata_form_12', raise_if_not_found=True)
        tbl1 = False
        for o in self:
            tbl1 = o.id
        result = {
            'name': '3rd class',
            'view_type': 'form',
            'res_model': 'dharmakata',
            'view_id': ac,
            'context': {'default_id_tbl1': tbl1},
            'type': 'ir.actions.act_window',
            'view_mode': 'form'
        }
        return result

    @api.multi
    def open_fourth_class(self):
        ac = self.env['ir.model.data'].xmlid_to_res_id('agni_loading_dharma_bharpai.bharpai_form_12', raise_if_not_found=True)
        tbl1 = False
        for o in self:
            tbl1 = o.id
        result = {
            'name': '4th class',
            'view_type': 'form',
            'res_model': 'bharpai',
            'view_id': ac,
            'context': {'default_id_sb': tbl1},
            'type': 'ir.actions.act_window',
            'view_mode': 'form'
        }
        return result


class loadingslip(models.Model):
    _name='loadingslip'
    _description='loading information'
    _inherits={'sale.order':'id_so'}
    id_so=fields.Many2one('sale.order')
    truck_no=fields.Integer(string='Truck Number')
    # name1=fields.Char(string="Name")


class dharmakata(models.Model):
    _name='dharmakata'
    sale_o=fields.Many2one('sale.order',string='Sale Order')
    bag=fields.Integer(string='No. of Bags')
    b_wt=fields.Char(string='Weight Before Loading')
    a_wt = fields.Char(string='Weight After Loading',compute = '_value_gratio')

    @api.depends('bag')
    def _value_gratio(self):
        # if self.sales > 0:
        self.a_wt = float(self.bag) * 50


class bharpai(models.Model):
    _name='bharpai'
    _inherits = {'sale.order': 'id_sb'}
    id_sb = fields.Many2one('sale.order')
    truck_no = fields.Integer(string='Truck Number')

view code:
<odoo>
  <data>
 <record id="sale.order_form" 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 name="arch" type="xml">
<xpath expr="/form/*" position="before">
    <header>
        <button name="open_second_class" type="object" string="loadingslip" />
        <button name="open_third_class" type="object" string="Dharmakata" />
        <button name="open_fourth_class" type="object" string="Bharpai" />
    </header>
</xpath>
  </field>
</record>


<record id="loadingslip_form_12" model="ir.ui.view">
        <field name="name">loadingslip.form.12</field>
        <field name="model">loadingslip</field>
        <field name="type">form</field>
<field name="arch" type="xml">
       <form string="Mymodule" duplicate="false">
                  <group>
                      <field name="state" invisible="1"/>

                      <field name="id_so"/>
                        <field name="partner_id" />
                      <field name="date_order" />
                      <field name="truck_no"/>
                  </group>
           <notebook>
                        <page string="Order Lines">
                            <field name="order_line" mode="tree,kanban">


                                <tree string="Sales Order Lines" editable="bottom" decoration-info="invoice_status=='to invoice'">

                                    <field name="product_id"/>


                                    <field name="product_uom_qty"/>

                                </tree>

                            </field>

                        </page>

                    </notebook>

              </form>
        </field>
    </record>


      <record id="loadingslip_tree_12" model="ir.ui.view">
        <field name="name">loadingslip.tree.12</field>
        <field name="model">loadingslip</field>
        <field name="type">form</field>
<field name="arch" type="xml">
       <tree string="Mymodule" export="false">
           <field name="name"/>
           <field name="partner_id" />
           <field name="date_order" />
           <field name="validity_date" />
              </tree>
        </field>
    </record>


      <record id="dharmakata_form_12" model="ir.ui.view">
        <field name="name">dharmakata.form.12</field>
        <field name="model">dharmakata</field>
        <field name="type">form</field>
<field name="arch" type="xml">
       <form string="Mymodule" duplicate="false">
           <group>
               <group>
                   <field name="sale_o"/>
                  </group>
               <group>
               </group>

                </group>
                  <group>
                      <group>

                      <field name="b_wt"/>
                  </group>
                      <group>
                          <field name="bag"/>
                      <field name="a_wt"/>
                  </group>
</group>
       </form>
        </field>
    </record>




      <record id="bharpai_form_12" model="ir.ui.view">
        <field name="name">bharpai.form.12</field>
        <field name="model">bharpai</field>
        <field name="type">form</field>
<field name="arch" type="xml">
       <form string="Mymodule" duplicate="false">
                  <group>

                      <field name="id_sb"/>



                        <field name="partner_id" />
                      <field name="date_order" />

                      <field name="truck_no"/>
                  </group>
           <notebook>
                        <page string="Order Lines">
                            <field name="order_line" mode="tree,kanban" attrs="{'readonly': [('state', 'in', ('done','cancel'))]}">

                                <tree string="Sales Order Lines" editable="bottom" decoration-info="invoice_status=='to invoice'">

                                    <field name="product_id"/>

                                    <field name="product_uom_qty" string="Ordered Qty" />

                                </tree>

                            </field>

                        </page>

                    </notebook>

              </form>
        </field>
    </record>




<act_window id="action_loadingslip" name ="loading_slip" res_model="loadingslip" view_mode="tree,form"/>
      <menuitem id ="menu_loadingslip"  parent="sale.menu_sale_invoicing" sequence="20" action="action_loadingslip"/>

  </data>
</odoo>