[FIX] sale_coupon: generate only one coupon per program per SO

Before this commit:
Everytime the SO met a 'on_next_order' program requirements, a coupon would be
generated on that SO. Even if there was already a coupon for that program set to
'expired', for instance when the SO did not met the requirements anymore.

Now:
When the requirements are met, it checks that an `expired` coupon does not
already exist.
It it does, it will set that coupon back to `reserved` state.

Step to reproduce:
  - Create a program on `next_order` with requirements, eg $300 amount
  - Create a SO and add $300 worth of products
  - Update promotion, a coupon is created and set to `reserved`
  - Now update the SO to go below $300
  - Update promotion, the coupon is set to `expired` as it should
  - Update again the SO to meet the $300 requirements again
  - Update promotion, a new `reserved` coupon is generated instead of updating
    the state of the previous one from `expired` to `reserved`

Closes #3062

models/sale_order.py

@@ -216,13 +216,24 @@ class SaleOrder(models.Model):
         self.write({'order_line': [(0, False, value) for value in self._get_reward_line_values(program)]})
 
     def _create_reward_coupon(self, program):
-        coupon = self.env['sale.coupon'].create({
-            'program_id': program.id,
-            'state': 'reserved',
-            'partner_id': self.partner_id.id,
-            'order_id': self.id,
-            'discount_line_product_id': program.discount_line_product_id.id
-        })
+        # if there is already a coupon that was set as expired, reactivate that one instead of creating a new one
+        coupon = self.env['sale.coupon'].search([
+            ('program_id', '=', program.id),
+            ('state', '=', 'expired'),
+            ('partner_id', '=', self.partner_id.id),
+            ('order_id', '=', self.id),
+            ('discount_line_product_id', '=', program.discount_line_product_id.id),
+        ], limit=1)
+        if coupon:
+            coupon.write({'state': 'reserved'})
+        else:
+            coupon = self.env['sale.coupon'].create({
+                'program_id': program.id,
+                'state': 'reserved',
+                'partner_id': self.partner_id.id,
+                'order_id': self.id,
+                'discount_line_product_id': program.discount_line_product_id.id
+            })
         self.generated_coupon_ids |= coupon
         return coupon