diff --git a/src/shared/db/__tests__/entity.spec.ts b/src/shared/db/__tests__/entity.spec.ts
new file mode 100644
index 0000000000..97651c1b7a
--- /dev/null
+++ b/src/shared/db/__tests__/entity.spec.ts
@@ -0,0 +1,29 @@
+import { TransactionOnchainEntity } from 'src/subdomains/lightning/entities/transaction-onchain.entity';
+
+// Pins "useDefineForClassFields": false in tsconfig.json. With define semantics (the default
+// for target >= es2022), fresh entities carry own undefined class fields, so the
+// Object.assign(dbEntity, freshEntity) update pattern wipes the loaded row's id and
+// save() inserts a duplicate row instead of updating (issue #205).
+describe('entity class-field semantics', () => {
+  it('fresh entity carries no own id property', () => {
+    const fresh = new TransactionOnchainEntity();
+
+    expect(Object.getOwnPropertyNames(fresh)).not.toContain('id');
+  });
+
+  it('Object.assign of a fresh entity onto a loaded row preserves the row id', () => {
+    const dbRow = new TransactionOnchainEntity();
+    dbRow.id = 42;
+    dbRow.transaction = 'txid';
+    dbRow.amount = 1;
+
+    const fresh = new TransactionOnchainEntity();
+    fresh.transaction = 'txid';
+    fresh.amount = 2;
+
+    Object.assign(dbRow, fresh);
+
+    expect(dbRow.id).toBe(42);
+    expect(dbRow.amount).toBe(2);
+  });
+});
diff --git a/tsconfig.json b/tsconfig.json
index 4fcd093082..ba136405e2 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -7,6 +7,7 @@
     "experimentalDecorators": true,
     "allowSyntheticDefaultImports": true,
     "target": "es2022",
+    "useDefineForClassFields": false,
     "sourceMap": true,
     "outDir": "./dist",
     "baseUrl": "./",