Spot the Bug - Typescript

Spot the Bug - Typescript

Recently, we faced an intriguing bug in one of our backend projects. The code below is a simplified version of the original code and it executes the following processes:

1. Fetch all templates

2. Fetch all customer ids

3. Generate a concrete value for each customer id and template

4. Save the concrete values in the database

However, the database throws an error regarding a unique constraint violation for the concrete values. Can you figure out what's causing the problem?

This is the SQL code:

1CREATE TABLE "template" (
2  "id" TEXT NOT NULL,
3  "startTime" TIMESTAMP(3) NOT NULL,
4  "price" DOUBLE PRECISION NOT NULL,
5  "listId" TEXT NOT NULL,
6
7  CONSTRAINT "template_pkey" PRIMARY KEY ("id")
8)
9
10CREATE TABLE "concrete" (
11    "id" TEXT NOT NULL,
12    "listId" CITEXT NOT NULL,
13    "startTime" TIMESTAMP(3) NOT NULL,
14    "price" DOUBLE PRECISION NOT NULL,
15    "customerId" CITEXT NOT NULL,
16
17    CONSTRAINT "concrete_pkey" PRIMARY KEY ("id")
18);
19
20CREATE UNIQUE INDEX "concrete_id_startTime_listId_key" ON "concrete"("id", "startTime", "listId");

And the following Typescript code:

1interface Template {
2  id: string;
3  startTime: Date;
4  price: number;
5  listId: string;
6}
7
8interface Concrete {
9  id: string;
10  startTime: Date;
11  price: number;
12  listId: string;
13  customerId: string;
14}
15
16async function getAllTemplates(): Promise<Template[]> {
17  return query('SELECT * FROM template');
18}
19
20async function getAllCustomerIds(): Promise<string[]> {
21  return Promise.resolve(['1', '2']); // no query here for simplicity
22}
23
24async function createConcreteValues(values: Omit<Concrete, 'id'>[]): Promise<void> {
25  // insert values into DB
26}
27
28async function applyTemplates() {
29  const allTemplates = await getAllTemplates();
30  const allCustomerIds = await getAllCustomerIds();
31
32  const concreteValues: Omit<Concrete, 'id'>[] = allTemplates.flatMap((template) => {
33    return allCustomerIds.map((customerId) => ({
34      ...template,
35      customerId,
36    });
37  };
38
39  // this call will throw a unique constraint violation exception; why?
40  await createConcreteValues(concreteValues);
41}

When generating the concreteValues array, all properties of a template, including their id, are fetched. Consequently, for each customerId, the template's id values are duplicated. Storing these duplicates triggers a unique constraint violation. The bug can be fixed in several ways but the primary learning here involves Typescript:

An object can comply with an interface and yet include properties that the interface doesn't dictate – and the compiler won't object.