Ensure workorder sequence is applied on new workorders

Test sequence is applied for multiple operations
Rewrite _reset_work_order_sequence in a more pythonic way
Change _order of mrp.workorder to use sequence
Set sequence on existing workorders after module install

mrp_workorder_sequence/models/mrp_production.py

@@ -9,12 +9,15 @@ class MrpProduction(models.Model):
 
     def _reset_work_order_sequence(self):
         for rec in self:
-            current_sequence = 1
-            for work in rec.workorder_ids:
-                work.sequence = current_sequence
-                current_sequence += 1
+            for current_seq, work in enumerate(rec.workorder_ids, 1):
+                work.sequence = current_seq
 
     def _create_workorder(self):
-        res = super()._create_workorder()
+        # Bypass sequence assignation on create and make sure there is no gap
+        # using _reset_work_order_sequence
+        res = super(
+            MrpProduction,
+            self.with_context(_bypass_sequence_assignation_on_create=True),
+        )._create_workorder()
         self._reset_work_order_sequence()
         return res

mrp_workorder_sequence/models/mrp_workorder.py

@@ -1,10 +1,42 @@
 # Copyright 2019-20 ForgeFlow S.L. (https://www.forgeflow.com)
 # License LGPL-3.0 or later (https://www.gnu.org/licenses/lgpl.html).
 
-from odoo import fields, models
+from odoo import api, fields, models
 
 
 class MrpWorkOrder(models.Model):
     _inherit = "mrp.workorder"
+    _order = "production_id, sequence, id"
 
     sequence = fields.Integer()
+
+    def _assign_sequence_on_create(self, values_list):
+        """Assign sequence number for manually added operations"""
+        new_wos_production_ids_without_seq = {
+            vals["production_id"] for vals in values_list if not vals.get("sequence")
+        }
+        if new_wos_production_ids_without_seq:
+            max_seq_by_production = self.read_group(
+                [("production_id", "in", list(new_wos_production_ids_without_seq))],
+                ["sequence:max", "production_id"],
+                ["production_id"],
+            )
+            max_seq_by_prod_id = {
+                res["production_id"][0]: res["sequence"]
+                for res in max_seq_by_production
+            }
+            for values in values_list:
+                prod_id = values["production_id"]
+                values_seq = values.get("sequence")
+                max_seq = max_seq_by_prod_id.setdefault(prod_id, 0)
+                if values_seq and values_seq > max_seq:
+                    max_seq_by_prod_id[prod_id] = values_seq
+                    continue
+                max_seq_by_prod_id[prod_id] += 1
+                values["sequence"] = max_seq_by_prod_id[prod_id]
+
+    @api.model_create_multi
+    def create(self, values_list):
+        if not self.env.context.get("_bypass_sequence_assignation_on_create"):
+            self._assign_sequence_on_create(values_list)
+        return super().create(values_list)