refactor(currencies): keep the exchanged amounts display rule in one place

Account.displayed_exchange_currency holds the account/currency precedence for transactions, and shown_exchange_currency() expresses the same rule as a query expression for the insights and account totals, replacing the per-module checks.

app/apps/accounts/models.py

@@ -199,6 +199,19 @@ class Account(SharedObject):
         user = get_current_user()
         return self.untracked_by.filter(pk=user.pk).exists()
 
+    @property
+    def displayed_exchange_currency(self):
+        """The exchange currency to show amounts in, or None when they're hidden.
+
+        The account's exchange currency takes precedence over its currency's, so
+        whichever one provides it also decides whether it's shown.
+        """
+        if self.exchange_currency_id:
+            return self.exchange_currency if self.show_exchanged_amounts else None
+        if self.currency.show_exchanged_amounts:
+            return self.currency.exchange_currency
+        return None
+
     def clean(self):
         super().clean()
         if self.exchange_currency == self.currency:

app/apps/currencies/utils/display.py

@@ -0,0 +1,13 @@
+from django.db.models import Case, F, When
+
+
+def shown_exchange_currency(path):
+    """Query expression for the exchange currency id at ``path`` (an account or a
+    currency lookup, such as ``"account__currency__"``), or NULL when that object
+    hides its exchanged amounts."""
+    return Case(
+        When(
+            **{f"{path}show_exchanged_amounts": True},
+            then=F(f"{path}exchange_currency"),
+        )
+    )

app/apps/insights/utils/month_by_month.py

@@ -8,6 +8,7 @@ from django.utils import timezone
 
 from apps.currencies.models import Currency
 from apps.currencies.utils.convert import convert
+from apps.currencies.utils.display import shown_exchange_currency
 from apps.transactions.models import Transaction
 
 
@@ -94,8 +95,7 @@ def get_month_by_month_data(year=None, group_by="categories"):
             "account__currency__decimal_places",
             "account__currency__prefix",
             "account__currency__suffix",
-            "account__currency__exchange_currency",
-            "account__currency__show_exchanged_amounts",
+            exchange_currency_id=shown_exchange_currency("account__currency__"),
         )
         .annotate(
             expense_total=Coalesce(
@@ -140,12 +140,7 @@ def get_month_by_month_data(year=None, group_by="categories"):
         item_name = metric[name_field]
         month = metric["reference_date__month"]
         currency_id = metric["account__currency"]
-        # Currencies can hide their exchanged amounts
-        exchange_currency_id = (
-            metric["account__currency__exchange_currency"]
-            if metric["account__currency__show_exchanged_amounts"]
-            else None
-        )
+        exchange_currency_id = metric["exchange_currency_id"]
 
         # Use a consistent key for None (uncategorized/untagged/no entity)
         item_key = item_id if item_id is not None else "__none__"

app/apps/insights/utils/overview.py

@@ -6,6 +6,7 @@ from django.db.models.functions import Coalesce
 
 from apps.currencies.models import Currency
 from apps.currencies.utils.convert import convert
+from apps.currencies.utils.display import shown_exchange_currency
 from apps.transactions.models import Transaction
 
 # Grouping levels the overview can be built from. Any ordering of these keys is
@@ -25,8 +26,6 @@ CURRENCY_FIELDS = (
     "account__currency__decimal_places",
     "account__currency__prefix",
     "account__currency__suffix",
-    "account__currency__exchange_currency",
-    "account__currency__show_exchanged_amounts",
 )
 
 # Aggregated amounts, plus the totals derived from them. Every one of these is
@@ -80,7 +79,12 @@ def _sum_of(transaction_type, is_paid):
 def _aggregate(transactions_queryset, group_fields, name_field):
     """Sum income/expense per currency, grouped by the given fields."""
     return (
-        transactions_queryset.values(*group_fields, name_field, *CURRENCY_FIELDS)
+        transactions_queryset.values(
+            *group_fields,
+            name_field,
+            *CURRENCY_FIELDS,
+            exchange_currency_id=shown_exchange_currency("account__currency__"),
+        )
         .annotate(
             expense_current=_sum_of(Transaction.Type.EXPENSE, is_paid=True),
             expense_projected=_sum_of(Transaction.Type.EXPENSE, is_paid=False),
@@ -122,14 +126,9 @@ def _build_currency_data(metric, showing):
         "total": currency_data[total],
     }
 
-    if (
-        metric["account__currency__exchange_currency"]
-        and metric["account__currency__show_exchanged_amounts"]
-    ):
+    if metric["exchange_currency_id"]:
         from_currency = Currency.objects.get(id=metric["account__currency"])
-        exchange_currency = Currency.objects.get(
-            id=metric["account__currency__exchange_currency"]
-        )
+        exchange_currency = Currency.objects.get(id=metric["exchange_currency_id"])
 
         exchanged = {}
         for field in AGGREGATED_FIELDS + TOTAL_FIELDS:

app/apps/insights/utils/year_by_year.py

@@ -7,6 +7,7 @@ from django.db.models.functions import Coalesce
 
 from apps.currencies.models import Currency
 from apps.currencies.utils.convert import convert
+from apps.currencies.utils.display import shown_exchange_currency
 from apps.transactions.models import Transaction
 
 
@@ -83,8 +84,7 @@ def get_year_by_year_data(group_by="categories"):
             "account__currency__decimal_places",
             "account__currency__prefix",
             "account__currency__suffix",
-            "account__currency__exchange_currency",
-            "account__currency__show_exchanged_amounts",
+            exchange_currency_id=shown_exchange_currency("account__currency__"),
         )
         .annotate(
             expense_total=Coalesce(
@@ -127,12 +127,7 @@ def get_year_by_year_data(group_by="categories"):
         item_name = metric[name_field]
         year = metric["reference_date__year"]
         currency_id = metric["account__currency"]
-        # Currencies can hide their exchanged amounts
-        exchange_currency_id = (
-            metric["account__currency__exchange_currency"]
-            if metric["account__currency__show_exchanged_amounts"]
-            else None
-        )
+        exchange_currency_id = metric["exchange_currency_id"]
 
         # Use a consistent key for None (uncategorized/untagged/no entity)
         item_key = item_id if item_id is not None else "__none__"

app/apps/transactions/models.py

@@ -573,49 +573,33 @@ class Transaction(OwnedObject):
         super().delete(*args, **kwargs)
 
     def exchanged_amount(self):
-        if self.account.exchange_currency:
-            converted_amount, prefix, suffix, decimal_places = convert(
-                self.amount,
-                to_currency=self.account.exchange_currency,
-                from_currency=self.account.currency,
-                date=self.date,
-            )
-            if converted_amount:
-                return {
-                    "amount": converted_amount,
-                    "prefix": prefix,
-                    "suffix": suffix,
-                    "decimal_places": decimal_places,
-                }
-        elif self.account.currency.exchange_currency:
-            converted_amount, prefix, suffix, decimal_places = convert(
-                self.amount,
-                to_currency=self.account.currency.exchange_currency,
-                from_currency=self.account.currency,
-                date=self.date,
-            )
-            if converted_amount:
-                return {
-                    "amount": converted_amount,
-                    "prefix": prefix,
-                    "suffix": suffix,
-                    "decimal_places": decimal_places,
-                }
-
-        return None
+        return self._exchanged_to(
+            self.account.exchange_currency or self.account.currency.exchange_currency
+        )
 
     def displayed_exchanged_amount(self):
-        """The exchanged amount to display, or None when it's hidden.
+        """The exchanged amount to display, or None when it's hidden."""
+        return self._exchanged_to(self.account.displayed_exchange_currency)
 
-        The account's exchange currency takes precedence over its currency's, so
-        whichever one provides it also decides whether it's shown.
-        """
-        if self.account.exchange_currency_id:
-            show = self.account.show_exchanged_amounts
-        else:
-            show = self.account.currency.show_exchanged_amounts
+    def _exchanged_to(self, exchange_currency):
+        if not exchange_currency:
+            return None
 
-        return self.exchanged_amount() if show else None
+        converted_amount, prefix, suffix, decimal_places = convert(
+            self.amount,
+            to_currency=exchange_currency,
+            from_currency=self.account.currency,
+            date=self.date,
+        )
+        if converted_amount:
+            return {
+                "amount": converted_amount,
+                "prefix": prefix,
+                "suffix": suffix,
+                "decimal_places": decimal_places,
+            }
+
+        return None
 
     def __str__(self):
         type_display = self.get_type_display()

app/apps/transactions/utils/calculations.py

@@ -6,6 +6,7 @@ from django.db.models.functions import Coalesce
 
 from apps.transactions.models import Transaction
 from apps.currencies.utils.convert import convert
+from apps.currencies.utils.display import shown_exchange_currency
 from apps.currencies.models import Currency
 
 
@@ -404,8 +405,7 @@ def calculate_account_totals(transactions_queryset, ignore_empty=False):
         "account__currency__decimal_places",
         "account__currency__prefix",
         "account__currency__suffix",
-        "account__exchange_currency",
-        "account__show_exchanged_amounts",
+        exchange_currency_id=shown_exchange_currency("account__"),
     ).annotate(
         expense_current=Coalesce(
             Sum(
@@ -472,9 +472,8 @@ def calculate_account_totals(transactions_queryset, ignore_empty=False):
         account_id = total["account"]
         currency = Currency.objects.get(id=total["account__currency__id"])
         exchange_currency = (
-            Currency.objects.get(id=total["account__exchange_currency"])
-            if total["account__exchange_currency"]
-            and total["account__show_exchanged_amounts"]
+            Currency.objects.get(id=total["exchange_currency_id"])
+            if total["exchange_currency_id"]
             else None
         )