How to stub EntityManager and Connection in TypeORM with Jest

Question

I got an app on NestJS in Typescript using TypeORM and unit-tests written with Jest. I have a function that uses transactions like this:

async createMany(users: User[]) {
  await this.connection.transaction(async manager => {
    await manager.save(users[0]);
    await manager.save(users[1]);
  });
}

That's an example from NestJS docs. I do it roughly in the same way via this.connection.transaction but the business-logic is different.

The thing is I want to make a unit-test to test this service function. So I need to somehow mock both this.connection and its manager. Or at least the manager. I'm not sure how to do it using Jest. I can't create a manager without a connection. I can't create a mock connection with no manager to return inside it.

Using both TypeORM and Jest is standard in NestJS. There have to be a way to write unit-tests with transactions. But I am not sure how to do it.

Note that I am asking about unit-test mocking ORM. Not integration tests that would directly use a testing db instance.

Answer 1

You would need to create a TestingModule with a mocked Connection. Something likes this:

import { Test } from "@nestjs/testing";

describe("UsersService", () => {
    let usersService;
    let connection;
    const mockConnection = () => ({
        transaction: jest.fn()
    });

    beforeEach(async () => {
        const module = await Test.createTestingModule({
            providers: [
                UsersService,
                {
                    provide: Connection,
                    useFactory: mockConnection
                }
            ],
        }).compile();

        usersService = await module.get<UsersService>(UsersService); 
        connection = await modle.get<Connection>(Connection);
    });

    describe("some tests", () => {
        it("should test something", async () => {
            const someMockedUsers = [/* some users */];
            const mockedManager = {
                save: jest.fn()
            }
            connection.transaction.mockImplementation((cb) => {
                cb(mockedManager);
            });

            await userService.createMany(someMockedUsers);

            expect(connection.transaction).toHaveBeenCalled();
            expect(mockedManager.save).toHaveBeenCalledTimes(2);
            // ...

        });
    });


});