1. master model

               from odoo import models, fields, api


               class document_entry(models.Model):
                    _name = 'csms.document_entry'
                    _description = 'csms.document_entry'

                    company_id = fields.Many2one('res.company', string='Branch', required=True)
                    country_id = fields.Many2one('csms.country', string='Country', required=True)
                    customer_id = fields.Many2one('res.partner', string='Customer', required=True)
                    email = fields.Char()
                  
                  
                    document_item_ids = fields.One2many(
                        'csms.document_item',
                        'entry_id',
                        string='Documents'
                    )

                    

1. note document_item_ids
2. entry_id is relation id in the model document_item
3. document_item_ids is One2many in master model

2. line item model

                                    
                  from odoo import models, fields, api


                  class document_item(models.Model):
                      _name = 'csms.document_item'
                      _description = 'csms.document_item'

                    
                      document_name = fields.Char()
                      university = fields.Char()
                      rate = fields.Char()
                      quantity = fields.Char()
                      total = fields.Char()   
                      entry_id = fields.Many2one(
                              'csms.document_entry'
                      )

                  

entry_id is Many2one

3. menu

                              <menuitem name="csms" id="csms.menu_root"/>

                              <!-- menu categories -->

                              <menuitem name="Activities" id="csms.menu_1" parent="csms.menu_root"/>
                             
                              <!-- actions -->

                              <menuitem name="Documents" id="csms.menu_1_list" parent="csms.menu_1"
                                        action="document_entry_action_window"/>

                      

4. action window

                        <record model="ir.actions.act_window" id="document_entry_action_window">
                          <field name="name">Document Entry</field>
                          <field name="res_model">csms.document_entry</field>
                          <field name="view_mode">tree,form</field>
                        </record>
                      

5. add permission to security

E:\odoo\16\server\addons\csms\security\ir.model.access.csv

                     access_csms_document_entry,csms.document_entry,model_csms_document_entry,base.group_user,1,1,1,1
                      access_csms_document_item,csms.document_item,model_csms_document_item,base.group_user,1,1,1,1
                      

1. Master UI customization

                        <record id="document_entry_view_form" model="ir.ui.view">
                              <field name="name">csms.document_entry</field>
                              <field name="model">csms.document_entry</field>
                              <field name="arch" type="xml">
                <form string="Planning">
                   
                    <sheet>
                        <group>
                            <group>
                                  <field name="company_id"/>
                                <field name="country_id"/>
                                
                              </group>
                              <group>
                                  
                                  <field name="customer_id"/>
                                
                              </group>
                        </group>
                        <notebook>
                            <page string="Activities To Generate">
                                <field name="document_item_ids" nolabel="1">
                                    <tree>
                                        <field name="document_name" />
                                        <field name="university"/>
                                        <field name="rate"/>
                                        <field name="quantity"/>
                                        <field name="total" />
                                       
                                    </tree>
                                </field>
                            </page>
                        </notebook>
                    </sheet>
                </form>
            </field>
        </record>

                      

1. note sheet, group , nested group, notebook

2. line item form

                    <record id="document_item_view_form" model="ir.ui.view">
                        <field name="name">csms.document_item</field>
                        <field name="model">csms.document_item</field>
                        <field name="arch" type="xml">
                            <form string="Activity">
                              
                                <sheet>
                                    <group>
                                        <field name="document_name"/>
                                        <field name="university"/>
                                      
                                    </group>
                                </sheet>
                            </form>
                        </field>
                    </record>