Mocking Database Queries in Jest for API Testing

When building APIs that interact with databases, it's crucial to test the API logic in isolation from the actual database. Jest provides powerful mocking capabilities that allow us to simulate database query results, ensuring that our API controllers and services behave as expected under various scenarios. This challenge focuses on creating effective mocks for database query functions.

Problem Description

Your task is to write Jest tests for a hypothetical UserService that retrieves user data from a database. The UserService relies on a DatabaseClient (which we will mock) to execute queries. You need to create mocks for the DatabaseClient's query methods to simulate different database responses, including successful retrieval of data, cases where no data is found, and potential database errors.

Key Requirements:

• Mock DatabaseClient: Create a Jest mock for the DatabaseClient class.
• Mock Query Methods: Specifically mock the query method of the DatabaseClient.
• Simulate Different Scenarios:
  • Successfully return a list of users.
  • Return an empty array when no users are found.
  • Simulate a database error (e.g., by throwing an exception).
• Test UserService: Write tests for the UserService's getUsers and getUserById methods, ensuring they correctly handle the mocked database responses.

Expected Behavior:

• When userService.getUsers() is called and the database mock returns users, the service should return that list of users.
• When userService.getUsers() is called and the database mock returns an empty array, the service should return an empty array.
• When userService.getUserById(id) is called and the database mock returns a specific user, the service should return that user.
• When userService.getUserById(id) is called and the database mock returns undefined or null (representing no user found), the service should return undefined or null.
• When the database mock throws an error during a query, the UserService methods should propagate that error.

Edge Cases to Consider:

• What happens if the DatabaseClient's query method is called with unexpected arguments? (While not strictly required to test for invalid arguments to the mock itself, consider how your UserService handles potentially incorrect data from the "database").

Examples

Example 1: Successful User Retrieval

Let's assume you have a DatabaseClient and UserService defined as follows (these will be the targets of your tests, not what you write for the challenge):

// --- Target Code (you will test this) ---

interface User {
  id: number;
  name: string;
  email: string;
}

class DatabaseClient {
  async query(sql: string): Promise<any[]> {
    // Actual database interaction logic (to be mocked)
    throw new Error("Not implemented");
  }
}

class UserService {
  private dbClient: DatabaseClient;

  constructor(dbClient: DatabaseClient) {
    this.dbClient = dbClient;
  }

  async getUsers(): Promise<User[]> {
    const users = await this.dbClient.query("SELECT * FROM users");
    return users;
  }

  async getUserById(id: number): Promise<User | undefined> {
    const users = await this.dbClient.query(`SELECT * FROM users WHERE id = ${id}`);
    return users.length > 0 ? users[0] : undefined;
  }
}

// --- End Target Code ---

Input to Jest Test (Mock Configuration):

We want to mock DatabaseClient so that its query method, when called with SELECT * FROM users, returns a specific array of users.

// Inside your Jest test file (.test.ts)

// Mock the DatabaseClient
const mockQuery = jest.fn();
const mockDbClient = {
  query: mockQuery,
};

// Configure the mock for a successful response
mockQuery.mockResolvedValue([
  { id: 1, name: "Alice", email: "alice@example.com" },
  { id: 2, name: "Bob", email: "bob@example.com" },
]);

// Instantiate UserService with the mock client
const userService = new UserService(mockDbClient as any); // Type assertion for simplicity in example

Calling the UserService method:

const users = await userService.getUsers();

Expected Output from userService.getUsers():

[
  { "id": 1, "name": "Alice", "email": "alice@example.com" },
  { "id": 2, "name": "Bob", "email": "bob@example.com" }
]

Explanation: The mockQuery function was configured to resolve with an array of user objects. When userService.getUsers() called this.dbClient.query(), it received this resolved value and returned it. The mockDbClient is an instance of our mock DatabaseClient.

Example 2: No Users Found

Input to Jest Test (Mock Configuration):

We want to mock DatabaseClient so that its query method returns an empty array.

// Inside your Jest test file (.test.ts)

const mockQuery = jest.fn();
const mockDbClient = {
  query: mockQuery,
};

// Configure the mock for an empty response
mockQuery.mockResolvedValue([]);

const userService = new UserService(mockDbClient as any);

Calling the UserService method:

const users = await userService.getUsers();

Expected Output from userService.getUsers():

[]

Explanation: The mockQuery was configured to resolve with an empty array. userService.getUsers() received this empty array and returned it.

Example 3: User Not Found by ID

Input to Jest Test (Mock Configuration):

We want to mock DatabaseClient so that its query method, when called for a specific ID, returns an empty array.

// Inside your Jest test file (.test.ts)

const mockQuery = jest.fn();
const mockDbClient = {
  query: mockQuery,
};

// Configure the mock to return empty for a specific ID query
mockQuery.mockResolvedValue([]); // Simulates not finding a user

const userService = new UserService(mockDbClient as any);

Calling the UserService method:

const user = await userService.getUserById(99); // Assuming user with ID 99 doesn't exist

Expected Output from userService.getUserById(99):

undefined

Explanation: When userService.getUserById(99) called this.dbClient.query("SELECT * FROM users WHERE id = 99"), the mock query function resolved with an empty array. The UserService logic then correctly returned undefined.

Example 4: Database Error

Input to Jest Test (Mock Configuration):

We want to mock DatabaseClient so that its query method throws an error.

// Inside your Jest test file (.test.ts)

const mockQuery = jest.fn();
const mockDbClient = {
  query: mockQuery,
};

// Configure the mock to reject with an error
const dbError = new Error("Database connection failed");
mockQuery.mockRejectedValue(dbError);

const userService = new UserService(mockDbClient as any);

Calling the UserService method (and expecting an error):

// In your test, you'd use expect().rejects
await expect(userService.getUsers()).rejects.toThrow("Database connection failed");

Expected Behavior: The userService.getUsers() call should throw the same error that the mockQuery rejected with.

Constraints

• Use Jest for mocking and testing.
• All code must be written in TypeScript.
• The DatabaseClient should be mocked using jest.fn() for its methods.
• Tests should cover the scenarios outlined in the examples.
• Focus on mocking the external dependency (DatabaseClient) so that the UserService can be tested in isolation.

Notes

• You will need to define the User, DatabaseClient, and UserService interfaces/classes as shown in the examples, or assume they are provided to you as the code you need to test. For this challenge, you are writing the tests and the mock setup.
• Consider using jest.mock('path/to/DatabaseClient') if you were mocking a module, but for this specific challenge, instantiating a mock object directly is sufficient and demonstrates the core mocking concept.
• Think about how to assert that the query method of your mock was called with the correct SQL statements. This is an important part of verifying that your UserService is interacting with the database client as expected.