purchase_usability: warning when price and/or delay is auto-updated following a qty change

It is similar to the feature present in sale_usability when the qty is
updated and the price unit changes

purchase_usability/purchase.py

@@ -3,8 +3,9 @@
 # @author Alexis de Lattre <alexis.delattre@akretion.com>
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
 
-from odoo import models, fields, api
+from odoo import models, fields, api, _
 from odoo.tools.misc import formatLang
+from odoo.tools import float_compare
 
 
 class PurchaseOrder(models.Model):
@@ -42,6 +43,51 @@ class PurchaseOrder(models.Model):
             if po.partner_ref:
                 name += ' (' + po.partner_ref + ')'
             if self.env.context.get('show_total_amount') and po.amount_total:
-                name += ': ' + formatLang(self.env, po.amount_untaxed, currency_obj=po.currency_id)
+                name += ': ' + formatLang(
+                    self.env, po.amount_untaxed, currency_obj=po.currency_id)
             result.append((po.id, name))
         return result
+
+
+class PurchaseOrderLine(models.Model):
+    _inherit = 'purchase.order.line'
+
+    @api.onchange('product_qty', 'product_uom')
+    def _onchange_quantity(self):
+        # When the user has manually set a price and/or planned_date
+        # he is often upset when Odoo changes it when he changes the qty
+        # So we add a warning...
+        res = {}
+        old_price = self.price_unit
+        old_date_planned = self.date_planned
+        super()._onchange_quantity()
+        new_price = self.price_unit
+        new_date_planned = self.date_planned
+        prec = self.env['decimal.precision'].precision_get('Product Price')
+        price_compare = float_compare(old_price, new_price, precision_digits=prec)
+        if price_compare or old_date_planned != new_date_planned:
+            res['warning'] = {
+                'title': _('Updates'),
+                'message': _(
+                    "Due to the update of the ordered quantity on line '%s', "
+                    "the following data has been updated using the supplier info "
+                    "of the product:"
+                    ) % self.name
+                }
+            if price_compare:
+                res['warning']['message'] += _(
+                    "\nOld price: %s\nNew price: %s") % (
+                        formatLang(
+                            self.env, old_price,
+                            currency_obj=self.order_id.currency_id),
+                        formatLang(
+                            self.env, new_price,
+                            currency_obj=self.order_id.currency_id))
+
+            if old_date_planned != new_date_planned:
+                res['warning']['message'] += _(
+                    "\nOld delivery date: %s\nNew delivery date: %s") % (
+                        old_date_planned,
+                        new_date_planned,
+                    )
+        return res