feat(rules): add rule function to fetch transactions totals and balance

app/apps/rules/utils/transactions.py

@@ -0,0 +1,58 @@
+import logging
+from decimal import Decimal
+
+from django.db.models import Sum, Value, DecimalField, Case, When, F
+from django.db.models.functions import Coalesce
+
+from apps.transactions.models import (
+    Transaction,
+)
+
+logger = logging.getLogger(__name__)
+
+
+class TransactionsGetter:
+    def __init__(self, **filters):
+        self.__queryset = Transaction.objects.filter(**filters)
+
+    @property
+    def sum(self):
+        return self.__queryset.aggregate(
+            total=Coalesce(
+                Sum("amount"), Value(Decimal("0")), output_field=DecimalField()
+            )
+        )["total"]
+
+    @property
+    def balance(self):
+        return abs(
+            self.__queryset.aggregate(
+                balance=Coalesce(
+                    Sum(
+                        Case(
+                            When(type=Transaction.Type.EXPENSE, then=-F("amount")),
+                            default=F("amount"),
+                            output_field=DecimalField(),
+                        )
+                    ),
+                    Value(Decimal("0")),
+                    output_field=DecimalField(),
+                )
+            )["balance"]
+        )
+
+    @property
+    def raw_balance(self):
+        return self.__queryset.aggregate(
+            balance=Coalesce(
+                Sum(
+                    Case(
+                        When(type=Transaction.Type.EXPENSE, then=-F("amount")),
+                        default=F("amount"),
+                        output_field=DecimalField(),
+                    )
+                ),
+                Value(Decimal("0")),
+                output_field=DecimalField(),
+            )
+        )["balance"]