feat: validates spend limit against subsidy balance (#477)

enterprise_access/apps/api/serializers/subsidy_access_policy.py

@@ -37,6 +37,13 @@ def policy_pre_write_validation(policy_instance_or_class, field_values_by_name):
     If a constraint occurs, raises a `ValidationError`.
     """
     violations = []
+
+    if isinstance(policy_instance_or_class, SubsidyAccessPolicy):
+        policy_instance_or_class.clean()
+    else:
+        instance = policy_instance_or_class(**field_values_by_name)
+        instance.clean()
+
     constraints = policy_instance_or_class.FIELD_CONSTRAINTS
 
     for field_name, new_value in field_values_by_name.items():

enterprise_access/apps/api/tests/test_serializers.py

@@ -86,6 +86,7 @@ def test_aggregates(
             'expiration_datetime': datetime.utcnow() + timedelta(days=1),
             'current_balance': starting_balance - redeemed,
             'is_active': True,
+            'total_deposits': starting_balance
         }
 
         # Create a test policy with a limit set to ``policy_spend_limit``.  Reminder: a value of 0 means no limit.
@@ -205,6 +206,7 @@ def test_get_subsidy_end_date(self, mock_subsidy_record, mock_transactions_for_l
             'expiration_datetime': subsidy_exp_date,
             'active_datetime': '2020-01-01 12:00:00Z',
             'current_balance': '1000',
+            'total_deposits': '1000',
         }
         serializer = SubsidyAccessPolicyCreditsAvailableResponseSerializer(
             [self.redeemable_policy],

enterprise_access/apps/api/v1/tests/test_subsidy_access_policy_views.py

@@ -10,6 +10,7 @@
 
 import ddt
 from django.conf import settings
+from django.core.exceptions import ValidationError
 from requests.exceptions import HTTPError
 from rest_framework import status
 from rest_framework.reverse import reverse
@@ -95,6 +96,8 @@ def setup_subsidy_mocks(self):
             'expiration_datetime': self.tomorrow,
             'current_balance': 4,
             'is_active': True,
+            'starting_balance': 4,
+            'total_deposits': 4,
         }
         subsidy_client_patcher = patch.object(
             SubsidyAccessPolicy, 'subsidy_client'
@@ -648,6 +651,43 @@ def test_update_views(self, is_patch, request_payload):
         expected_response.update(request_payload)
         self.assertEqual(expected_response, response.json())
 
+    def test_update_views_with_exceeding_spend_limit(self):
+        """
+        Test that the update and partial_update views can modify certain
+        fields of a policy record.
+        """
+        # Set the JWT-based auth to an operator.
+        self.set_jwt_cookie([
+            {'system_wide_role': SYSTEM_ENTERPRISE_OPERATOR_ROLE, 'context': str(TEST_ENTERPRISE_UUID)}
+        ])
+
+        policy_for_edit = PerLearnerSpendCapLearnerCreditAccessPolicyFactory(
+            enterprise_customer_uuid=self.enterprise_uuid,
+            display_name='old display_name',
+            spend_limit=5,
+            active=False,
+        )
+
+        request_payload = {
+            'description': 'the new description',
+            'display_name': 'new display_name',
+            'active': True,
+            'retired': True,
+            'catalog_uuid': str(uuid4()),
+            'subsidy_uuid': str(uuid4()),
+            'access_method': AccessMethods.ASSIGNED,
+            'spend_limit': 6,
+            'per_learner_spend_limit': 10000,
+        }
+
+        url = reverse(
+            'api:v1:subsidy-access-policies-detail',
+            kwargs={'uuid': str(policy_for_edit.uuid)}
+        )
+
+        with self.assertRaises(ValidationError):
+            self.client.patch(url, data=request_payload)
+
     @ddt.data(
         {
             'enterprise_customer_uuid': str(uuid4()),

enterprise_access/apps/subsidy_access_policy/constants.py

@@ -121,3 +121,9 @@ class MissingSubsidyAccessReasonUserMessages:
 SORT_BY_ENROLLMENT_COUNT = 'enrollment_count'
 
 GROUP_MEMBERS_WITH_AGGREGATES_DEFAULT_PAGE_SIZE = 10
+
+# Exceeding the spend_limit validation error
+VALIDATION_ERROR_SPEND_LIMIT_EXCEEDS_STARTING_BALANCE = "You cannot make this change, as the value of all budget \
+limits would exceed the funds available on the subsidy. Please double-check the subsidy’s initial value and any \
+adjustments, then ensure the budgets sum to an equal or lower amount. If you are trying to re-balance policies, \
+please reduce the value of one first, then proceed to increase the value of another"

enterprise_access/apps/subsidy_access_policy/models.py

@@ -38,6 +38,7 @@
     REASON_POLICY_EXPIRED,
     REASON_POLICY_SPEND_LIMIT_REACHED,
     REASON_SUBSIDY_EXPIRED,
+    VALIDATION_ERROR_SPEND_LIMIT_EXCEEDS_STARTING_BALANCE,
     AccessMethods,
     TransactionStateChoices
 )
@@ -298,6 +299,33 @@ def get_policy_class_by_type(cls, policy_type):
                 return policy_class
         return None
 
+    @property
+    def total_spend_limit_for_all_policies_associated_to_subsidy(self):
+        """
+        Sums the policies spend_limit excluding the db's instance of this policy
+        """
+        policy_balances = []
+        policy_balances.append(self.spend_limit or 0)
+        sibling_policies = SubsidyAccessPolicy.objects.filter(
+            enterprise_customer_uuid=self.enterprise_customer_uuid,
+            subsidy_uuid=self.subsidy_uuid,
+        ).exclude(uuid=self.uuid)
+
+        for sibling in sibling_policies:
+            policy_balances.append(sibling.spend_limit or 0)
+        return sum(policy_balances)
+
+    @property
+    def is_spend_limit_updated(self):
+        """
+        Checks if SubsidyAccessPolicy object exists in the database, and determines if the
+        database value of spend_limit differs from the current instance of spend_limit
+        """
+        if self._state.adding:
+            return False
+        record_from_db = SubsidyAccessPolicy.objects.get(uuid=self.uuid)
+        return record_from_db.spend_limit != self.spend_limit
+
     @property
     def is_assignable(self):
         """
@@ -309,6 +337,9 @@ def clean(self):
         """
         Used to help validate field values before saving this model instance.
         """
+        if self.is_spend_limit_updated:
+            if self.total_spend_limit_for_all_policies_associated_to_subsidy > self.subsidy_total_deposits():
+                raise ValidationError(f'{self} {VALIDATION_ERROR_SPEND_LIMIT_EXCEEDS_STARTING_BALANCE}')
         for field_name, (constraint_function, error_message) in self.FIELD_CONSTRAINTS.items():
             field = getattr(self, field_name)
             if not constraint_function(field):
@@ -403,6 +434,13 @@ def subsidy_balance(self):
         current_balance = self.subsidy_record().get('current_balance') or 0
         return int(current_balance)
 
+    def subsidy_total_deposits(self):
+        """
+        Returns total remaining balance for the associated subsidy ledger.
+        """
+        total_deposits = self.subsidy_record().get('total_deposits') or 0
+        return int(total_deposits)
+
     @property
     def spend_available(self):
         """