[FIX] gamification: Goal definition "sum"

Steps:
- Open Settings/Gamification Tools/Challenges
- Create a new challenge
    - Keep "Assign Challenge to" with default values
    - Need a domain that return some user
- Create and Edit a goal
    - Computation Mode: "Automatic: sum on a field"
    - Save & Close
- In reward Tab set "For Every Succeeding User"
- Save
- Start Challenge

Actual result:
- Traceback due to wrong aggregate value
- CHallenge is not started

Expected result:
- Challenge is started

opw-3740960

odoo/odoo@234db70d

closes odoo/odoo#154614

X-original-commit: aa0f617682a59c5ac285308000f35d44b75f04df
Signed-off-by: Rémy Voet (ryv) <ryv@odoo.com>

addons/gamification/models/gamification_goal.py

@@ -211,7 +211,7 @@ class Goal(models.Model):
 
                         if definition.computation_mode == 'sum':
                             field_name = definition.field_id.name
-                            res = Obj._read_group(domain, [], [field_name])
+                            res = Obj._read_group(domain, [], [f'{field_name}:{definition.computation_mode}'])
                             new_value = res[0][0] or 0.0
 
                         else:  # computation mode = count

addons/gamification/tests/test_challenge.py

@@ -151,6 +151,39 @@ class test_challenge(TestGamificationCommon):
             unchanged_goal_ids.user_id,
         )
 
+    def test_30_create_challenge_with_sum_goal(self):
+        challenge = self.env['gamification.challenge'].create({
+            'name': 'test',
+            'state': 'draft',
+            'user_domain': '[("active", "=", True)]', #Include all active users to get a least one participant
+            'reward_id': 1,
+        })
+
+        model = self.env['ir.model'].search([('model', '=', 'gamification.badge')])[0]
+        field = self.env['ir.model.fields'].search([('model', '=', 'gamification.badge'), ('name', '=', 'rule_max_number')])[0]
+
+        sum_goal = self.env['gamification.goal.definition'].create({
+            'name': 'test',
+            'computation_mode': 'sum',
+            'model_id': model.id,
+            'field_id': field.id
+        })
+
+        self.env['gamification.challenge.line'].create({
+            'challenge_id': challenge.id,
+            'definition_id': sum_goal.id,
+            'condition': 'higher',
+            'target_goal': 1
+        })
+
+        challenge.action_start()
+
+        self.assertEqual(
+            challenge.state,
+            'inprogress',
+            "Challenge failed to start",
+        )
+
 
 class test_badge_wizard(TestGamificationCommon):