Mock Prisma with Jest in NestJS

Concept:

• Define a type for the mock object
• Assign mock value in the provider

Let's say you have a NestJS service:

class TheServiceYouWantToTest {
  constructure(
    private readonly prisma: PrismaService // <- the Prisma service contains its client (models)
  ){}
}

To test TheServiceYouWantToTest, you might need to mock PrismaService.

This is the pattern in test code that I use for mock PrismaService:

type MockPrismaService = {
  model: {
    // define only the method I'll use in the implementation code
    findMany: jest.Mock
    ...
  }
}

describe('some sort of service', () => {
  let prisma: MockPrismaService

  beforeEach(() => {
    // create the test module with our service undertest and the mock service
    const mockPrismaService: MockPrismaService = {
      model: {
        findMany: jest.fn(),
        ...
      }
    }
    const module = await Test.createTestingModule({
      providers: [
        TheServiceYouWantToTest,
        {
          provide: PrismaService,
          useValue: mockPrismaService,
        }
      ],
    })

    // get the mock object that is already binding with the test module
    // the object type will be MockPrismaService
    prisma = module.get(PrismaService)
  })

  // and then we can ...

  // mock it
  prisma.model.findMany.mockResolvedValue(...)

  // spy it
  expect(prisma.model.findMany).toHaveBeenCalled()

  // without type error
})