[ADD] purchase_discount: add global discount functionality.

add discount button on purchase order page.
two ways to give discount in percentage and dollar.
check discount is valid or not.

Author: utpat-odoo

.gitignore

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+from . import models

purchase_discount/__init__.py

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+{
+    'author': 'Odoo S.A.',
+    'name': 'Purchase Discount',
+    'depends': ['purchase'],
+    'license': 'LGPL-3',
+    'data': [
+        'security/ir.model.access.csv',
+        'views/purchase_order_discount_views.xml',
+        'views/purchase_order_views.xml'
+    ],
+    'application': True,
+    'installable': True
+}

purchase_discount/__manifest__.py

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+from . import purchase_order_discount
+from . import purchase_order

purchase_discount/models/__init__.py

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+from odoo import models, fields
+
+
+class PurchaseOrder(models.Model):
+    _inherit = "purchase.order"
+
+    discount_id = fields.Many2one("purchase.order.discount")
+
+    def action_discount(self):
+        if not self.discount_id:
+            new_data = self.env["purchase.order.discount"].create(
+                {
+                    "discount_type": "percentage",
+                    "discount_in_value": 0,
+                    "discount_in_percentage": 0,
+                }
+            )
+            self.discount_id = new_data.id
+        return {
+            "type": "ir.actions.act_window",
+            "name": "Discount",
+            "res_model": "purchase.order.discount",
+            "view_mode": "form",
+            "view_id": self.env.ref(
+                "purchase_discount.purchase_order_discount_view_form"
+            ).id,
+            "res_id": self.discount_id.id,
+            "target": "new",
+        }

purchase_discount/models/purchase_order.py

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+from odoo import models, fields, api
+from odoo.exceptions import ValidationError
+
+
+class PurchaseOrderDiscount(models.Model):
+    _name = "purchase.order.discount"
+    _description = "Purchase Order Discount"
+
+    order_id = fields.One2many("purchase.order", "discount_id")
+    discount_type = fields.Selection(
+        selection=[
+            ("value", "$"),
+            ("percentage", "%"),
+        ],
+        default="percentage",
+    )
+    discount_in_value = fields.Float(string="Discount")
+    discount_in_percentage = fields.Float(compute="_compute_discount_percentage", store=True, readonly=True)
+
+    @api.depends("discount_type", "discount_in_value")
+    def _compute_discount_percentage(self):
+        for record in self:
+            if record.discount_type == "value" and record.order_id.amount_total > 0:
+                record.discount_in_percentage = (record.discount_in_value * 100) / (record.order_id.amount_total)
+            elif record.discount_type == "percentage":
+                record.discount_in_percentage = record.discount_in_value
+            else:
+                record.discount_in_percentage = 0.0
+
+    @api.constrains("discount_in_value", "discount_type", "order_id.amount_total")
+    def _check_discount_price(self):
+        for record in self:
+            if record.discount_type == "percentage":
+                if record.discount_in_value < 0 or record.discount_in_value > 100:
+                    raise ValidationError("discount value is not valid please check again")
+            if record.discount_type == "value":
+                if record.discount_in_value < 0 or record.discount_in_value > record.order_id.amount_total:
+                    raise ValidationError("discount value is not valid please check again")
+
+    def apply_discount(self):
+        self.order_id.order_line.discount = self.discount_in_percentage
+
+    def apply_cancle(self):
+        pass

purchase_discount/models/purchase_order_discount.py

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+"id","name","model_id:id","group_id:id","perm_read","perm_write","perm_create","perm_unlink"
+"access_purchase_order_discount","access_purchase_order_discount","model_purchase_order_discount","base.group_user",1,1,1,1

purchase_discount/security/ir.model.access.csv

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+<?xml version="1.0"?>
+<odoo>
+    <record id="purchase_order_discount_view_form" model="ir.ui.view">
+        <field name="name">purchase.order.discount.form</field>
+        <field name="model">purchase.order.discount</field>
+        <field name="arch" type="xml">
+            <form>
+                <sheet>
+                    <group>
+                        <field name="discount_type"/>
+                        <field name="discount_in_value"/>
+                        <field name="discount_in_percentage"/>
+                    </group>
+                </sheet>
+                <footer>
+                    <button name="apply_discount" type="object" string="Apply" class="oe_highlight" confirm="Are you sure?"/>
+                    <button name="apply_cancle" type="object" string="Cancle"/>
+                </footer>
+            </form>
+        </field>
+    </record>
+
+    <record id="action_open_discount" model="ir.actions.act_window">
+        <field name="name">Apply Discount</field>
+        <field name="res_model">purchase.order.discount</field>
+        <field name="view_mode">form</field>
+    </record>
+</odoo>

purchase_discount/views/purchase_order_discount_views.xml

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+<?xml version="1.0"?>
+<odoo>
+    <record id="purchase_order_form_inherit" model="ir.ui.view">
+        <field name="name">purchase.order.view.form.inherit</field>
+        <field name="model">purchase.order</field>
+        <field name="inherit_id" ref="purchase.purchase_order_form"/>
+        <field name="arch" type="xml">
+            <xpath expr="//*[hasclass('oe_subtotal_footer')]" position="replace">
+                <div class="d-flex flex-column align-items-end">
+                    <button name="action_discount" type="object" class="oe_highlight" string="Discount" invisible="state in ('purchase', 'cancel')"/>
+                    <group class="oe_subtotal_footer">
+                        <field name="tax_totals" widget="account-tax-totals-field" nolabel="1" readonly="1"/>
+                    </group>
+                </div>
+            </xpath>
+        </field>
+    </record>
+</odoo>