DWPF-1767 Improve new_finance_year command (#505)

Whilst trying to run the `new_finance_year` command, we ran into some
bad data that had made it's way into the database. What we found was a
used financial code with a NAC that had an invalid economic budget code.
To resolve the issue we fixed the data by changing the NAC's economic
budget code.

This commit extends the `new_finance_year` command to detect this issue
before changing the financial year. It also serves to document the issue
we faced.

forecast/admin.py

@@ -137,7 +137,7 @@ class FinancialCodeAdmin(AdminReadOnly):
         ("programme", RelatedDropdownFilter),
         ("analysis1_code", RelatedDropdownFilter),
         ("analysis2_code", RelatedDropdownFilter),
-        ("project_code,", RelatedDropdownFilter),
+        ("project_code", RelatedDropdownFilter),
     )
 
     def get_readonly_fields(self, request, obj=None):

forecast/management/commands/new_financial_year.py

@@ -7,6 +7,8 @@
     get_current_financial_year,
     get_year_display,
 )
+from forecast.models import FinancialCode
+from forecast.utils.import_helpers import VALID_ECONOMIC_CODE_LIST
 
 
 class Command(CommandWithUserCheck):
@@ -27,6 +29,11 @@ def run_command(self, message, command_name, *arg, **options):
         return True
 
     def handle_user(self, *args, **options):
+        try:
+            pre_new_financial_year_checks()
+        except NewFinancialYearError as err:
+            raise CommandError(str(err)) from err
+
         current_financial_year = get_current_financial_year()
         current_financial_year_display = get_year_display(current_financial_year)
         new_financial_year = current_financial_year + 1
@@ -68,3 +75,27 @@ def handle_user(self, *args, **options):
         self.stdout.write(
             self.style.SUCCESS(f"FFT ready for {new_financial_year_display} ")
         )
+
+
+class NewFinancialYearError(Exception):
+    pass
+
+
+def pre_new_financial_year_checks() -> None:
+    """Pre-flight checks for changing to a new financial year.
+
+    This function will raise `NewFinancialYearError` if any issues are found.
+
+    Checks:
+        - Look for used NACs with invalid economic budget codes.
+    """
+    # It's possible this needs to be scoped to a financial year in the future.
+    problem_nacs = FinancialCode.objects.exclude(
+        natural_account_code__economic_budget_code__in=VALID_ECONOMIC_CODE_LIST
+    )
+
+    if bool(problem_nacs):
+        problem_nac_ids = [str(nac.natural_account_code) for nac in problem_nacs]
+        raise NewFinancialYearError(
+            f"Possible problem NACs found: {", ".join(problem_nac_ids)}"
+        )