Unit testing

Unit testing aims to isolate a small portion (unit) of code and test it for logically predictable behaviors. It generally involves mocking objects or server responses to simulate real world behaviors. Some benefits to unit testing include:

• Quickly find and isolate bugs in code.
• Provides documentation for each module of code by way of indicating what certain code blocks should be doing.
• A helpful gauge that a refactor has gone well. The tests should still pass after code has been refactored.

In the context of Prisma, this generally means testing a function which makes database calls using Prisma Client.

A single test should focus on how your function logic handles different inputs (such as a null value or an empty list).

This means that you should aim to remove as many dependencies as possible, such as external services and databases, to keep the tests and their environments as lightweight as possible.

Prerequisites

This guide assumes you have the JavaScript testing library Jest and ts-jest already setup in your project.

Mocking Prisma Client

To ensure your unit tests are isolated from external factors you can mock Prisma Client, this means you get the benefits of being able to use your schema (type-safety), without having to make actual calls to your database when your tests are run.

This guide will cover two approaches to mocking Prisma Client, a singleton instance and dependency injection. Both have their merits depending on your use cases. To help with mocking Prisma Client the jest-mock-extended package will be used.

$npm install jest-mock-extended@2.0.4 --save-dev

jest.config.js
1module.exports = {
2  clearMocks: true,
3  preset: 'ts-jest',
4  testEnvironment: 'node',
+  setupFilesAfterEnv: ['<rootDir>/singleton.ts'],
6}

Example unit tests

A real world use case for unit testing Prisma might be a signup form. Your user fills in a form which calls a function, which in turn uses Prisma to make a call to your database.

All of the examples that follow use the following schema model:

schema.prisma
1model User {
2  id                       Int     @id @default(autoincrement())
3  email                    String  @unique
4  name                     String?
5  acceptTermsAndConditions Boolean
6}

The following unit tests will mock the process of

• Creating a new user
• Updating a users name
• Failing to create a user if terms are not accepted

The functions that use the dependency injection pattern will have the context injected (passed in as a parameter) into them, whereas the functions that use the singleton pattern will use the singleton instance of Prisma Client.

import { Context } from './context'

interface CreateUser {
  name: string
  email: string
  acceptTermsAndConditions: boolean
}

export async function createUser(user: CreateUser, ctx: Context) {
  if (user.acceptTermsAndConditions) {
    return await ctx.prisma.user.create({
      data: user,
    })
  } else {
    return new Error('User must accept terms!')
  }
}

interface UpdateUser {
  id: number
  name: string
  email: string
}

export async function updateUsername(user: UpdateUser, ctx: Context) {
  return await ctx.prisma.user.update({
    where: { id: user.id },
    data: user,
  })
}

The tests for each methodology are fairly similar, the difference is how the mocked Prisma Client is used.

The dependency injection example passes the context through to the function that is being tested as well as using it to call the mock implementation.

The singleton example uses the singleton client instance to call the mock implementation.

import { createUser, updateUsername } from '../functions-without-context'
import { prismaMock } from '../singleton'

test('should create new user ', async () => {
  const user = {
    id: 1,
    name: 'Rich',
    email: 'hello@prisma.io',
    acceptTermsAndConditions: true,
  }

  prismaMock.user.create.mockResolvedValue(user)

  await expect(createUser(user)).resolves.toEqual({
    id: 1,
    name: 'Rich',
    email: 'hello@prisma.io',
    acceptTermsAndConditions: true,
  })
})

test('should update a users name ', async () => {
  const user = {
    id: 1,
    name: 'Rich Haines',
    email: 'hello@prisma.io',
    acceptTermsAndConditions: true,
  }

  prismaMock.user.update.mockResolvedValue(user)

  await expect(updateUsername(user)).resolves.toEqual({
    id: 1,
    name: 'Rich Haines',
    email: 'hello@prisma.io',
    acceptTermsAndConditions: true,
  })
})

test('should fail if user does not accept terms', async () => {
  const user = {
    id: 1,
    name: 'Rich Haines',
    email: 'hello@prisma.io',
    acceptTermsAndConditions: false,
  }

  prismaMock.user.create.mockImplementation()

  await expect(createUser(user)).resolves.toEqual(
    new Error('User must accept terms!')
  )
})