fix: HL-1544 (#3500)

* fix: first instalment amount if two instalments

* feat: instalments in Django admin

* fix: instalments disabled by default

backend/benefit/calculator/admin.py

@@ -3,6 +3,7 @@
 from calculator.models import (
     Calculation,
     CalculationRow,
+    Instalment,
     PaySubsidy,
     PreviousBenefit,
     TrainingCompensation,
@@ -18,9 +19,15 @@ class CalculationRowInline(admin.StackedInline):
     list_filter = ("calculation",)
 
 
+class InstalmentInline(admin.TabularInline):
+    model = Instalment
+    extra = 0
+    readonly_fields = ("created_at",)
+
+
 class CalculationInline(admin.StackedInline):
     model = Calculation
-    inlines = (CalculationRowInline,)
+    inlines = (InstalmentInline, CalculationRowInline)
     show_change_link = True
     list_display = (
         "id",
@@ -38,10 +45,14 @@ class CalculationInline(admin.StackedInline):
 
 
 class CalculationAdmin(admin.ModelAdmin):
-    inlines = (CalculationRowInline,)
+    inlines = (
+        InstalmentInline,
+        CalculationRowInline,
+    )
     list_filter = ("application", "application__status", "application__company")
     list_display = (
         "id",
+        "application_id",
         "start_date",
         "end_date",
         "calculated_benefit_amount",

backend/benefit/calculator/rules.py

@@ -44,6 +44,7 @@ def __init__(self, calculation: Calculation):
         self.calculation = calculation
         self._row_counter = 0
         self.instalment_threshold = settings.INSTALMENT_THRESHOLD
+        self.first_instalment_limit = settings.FIRST_INSTALMENT_LIMIT
 
     def _get_change_days(
         self, pay_subsidies, training_compensations, start_date, end_date
@@ -172,13 +173,19 @@ def _calculate_instalment_amounts(
     ) -> list[tuple[int, decimal.Decimal, datetime.datetime]]:
         """Calculate the number of instalments and their amounts based on the total benefit.
         Returns a list of tuples containing (instalment_number, amount, due_date).
+        If the total benefit is less or equal to the instalment threshold, a single instalment is created.
+        If not, two instalments are created, with the amount of
+        the first instalment being equal to the FIRST_INSTALMENT_LIMIT,
+        and the second being equal to the rest of the total.
         """
         if total_benefit_amount <= self.instalment_threshold:
             return [(1, total_benefit_amount, timezone.now())]
 
-        second_instalment_amount = total_benefit_amount - self.instalment_threshold
+        first_instalment_amount = self.first_instalment_limit
+        second_instalment_amount = total_benefit_amount - first_instalment_amount
+
         return [
-            (1, self.instalment_threshold, timezone.now()),
+            (1, first_instalment_amount, timezone.now()),
             (
                 2,
                 second_instalment_amount,

backend/benefit/calculator/tests/test_calculator_api.py

@@ -707,6 +707,7 @@ def test_application_calculation_instalments(
     settings.PAYMENT_INSTALMENTS_ENABLED = True
     settings.INSTALMENT_THRESHOLD = 9600
     settings.SALARY_BENEFIT_NEW_MAX = 1500
+    settings.FIRST_INSTALMENT_LIMIT = 9000
 
     if has_subsidies is False:
         handling_application.pay_subsidies.all().delete()
@@ -742,12 +743,12 @@ def test_application_calculation_instalments(
         )
 
     if number_of_instalments == 2:
-        assert instalment_1.amount == decimal.Decimal(settings.INSTALMENT_THRESHOLD)
+        assert instalment_1.amount == decimal.Decimal(settings.FIRST_INSTALMENT_LIMIT)
         instalment_2 = handling_application.calculation.instalments.all()[1]
         assert (
             instalment_2.amount
             == handling_application.calculation.calculated_benefit_amount
-            - decimal.Decimal(settings.INSTALMENT_THRESHOLD)
+            - decimal.Decimal(settings.FIRST_INSTALMENT_LIMIT)
         )
 
         due_date = instalment_2.due_date

backend/benefit/helsinkibenefit/settings.py

@@ -180,6 +180,7 @@
     PAYMENT_INSTALMENTS_ENABLED=(bool, False),
     SALARY_BENEFIT_NEW_MAX=(int, 1500),
     INSTALMENT_THRESHOLD=(int, 9600),
+    FIRST_INSTALMENT_LIMIT=(int, 9000),
 )
 if os.path.exists(env_file):
     env.read_env(env_file)
@@ -570,3 +571,4 @@
 PAYMENT_INSTALMENTS_ENABLED = env.bool("PAYMENT_INSTALMENTS_ENABLED")
 SALARY_BENEFIT_NEW_MAX = env.int("SALARY_BENEFIT_NEW_MAX")
 INSTALMENT_THRESHOLD = env.int("INSTALMENT_THRESHOLD")
+FIRST_INSTALMENT_LIMIT = env.int("FIRST_INSTALMENT_LIMIT")