refactor: single bank operation projection and domain rules (WealthLedger/backend#168) (#189)

src/modules/banking/bank-cards/domain/bank-operation.rules.ts

@@ -0,0 +1,79 @@
+import { BankOperationKind } from './bank-operation-kind';
+
+export type OperationFields = {
+  cardId: string;
+  categoryId?: null | string;
+  kind: BankOperationKind;
+  originalExpenseId?: null | string;
+  targetCardId?: null | string;
+};
+
+export enum OperationFieldRule {
+  InvalidCombination = 'invalidCombination',
+  MissingOriginalExpense = 'missingOriginalExpense',
+  MissingTargetCard = 'missingTargetCard',
+  SameTargetCard = 'sameTargetCard',
+}
+
+function transferConflict(fields: OperationFields): null | OperationFieldRule {
+  if (!fields.targetCardId) {
+    return OperationFieldRule.MissingTargetCard;
+  }
+  if (fields.targetCardId === fields.cardId) {
+    return OperationFieldRule.SameTargetCard;
+  }
+  if (fields.categoryId || fields.originalExpenseId) {
+    return OperationFieldRule.InvalidCombination;
+  }
+  return null;
+}
+
+function expenseConflict(fields: OperationFields): null | OperationFieldRule {
+  if (fields.originalExpenseId) {
+    return OperationFieldRule.InvalidCombination;
+  }
+  // Expense without a category uses the user's default category in the service.
+  return null;
+}
+
+function refundConflict(fields: OperationFields): null | OperationFieldRule {
+  return fields.originalExpenseId
+    ? null
+    : OperationFieldRule.MissingOriginalExpense;
+}
+
+/**
+ * Pure kind/field compatibility rules for bank operations; no Prisma or Nest.
+ * Returns the first violated rule, or null when the combination is allowed.
+ */
+export function operationFieldConflict(
+  fields: OperationFields,
+): null | OperationFieldRule {
+  if (fields.kind === BankOperationKind.Transfer) {
+    return transferConflict(fields);
+  }
+  if (fields.targetCardId) return OperationFieldRule.InvalidCombination;
+  if (fields.kind === BankOperationKind.Expense) {
+    return expenseConflict(fields);
+  }
+  if (fields.categoryId) return OperationFieldRule.InvalidCombination;
+  if (fields.kind === BankOperationKind.Refund) {
+    return refundConflict(fields);
+  }
+  // Topup: no target, no category, no purchase link.
+  return fields.originalExpenseId
+    ? OperationFieldRule.InvalidCombination
+    : null;
+}
+
+export function operationKindChanged(
+  oldKind: string,
+  newKind: string,
+  oldOriginalExpenseId: null | string,
+  newOriginalExpenseId: null | string,
+): boolean {
+  return (
+    oldKind !== newKind ||
+    oldOriginalExpenseId !== newOriginalExpenseId
+  );
+}