Vitest Prisma Dual Query Mocking

Problem

When code uses both findFirst and findUnique on the same Prisma/ZenStack model, a single mock implementation doesn't work correctly. One query returns the mocked data while the other returns undefined, causing test failures.

Context / Trigger Conditions

Test error: "Cannot read properties of undefined (reading 'value')" or similar
Code uses db.model.findFirst({ where: {...} }) for one query
Code uses db.model.findUnique({ where: { composite_key: {...} } }) for another query
Both queries target the same model (e.g., userPreference)
Single mock like mockDb.model.findFirst = vi.fn().mockResolvedValue(data) doesn't cover both
Tests pass individually but fail when both queries run in same test

Example failing test:

// This mock only works for findFirst, not findUnique
mockDb.userPreference.findFirst = vi.fn().mockResolvedValue({
  userId,
  key: "email.notification.messaging",
  value: "true",
});

// This query works
const enabled = await db.userPreference.findFirst({ where: { userId, key: "..." } });

// This query returns undefined (not mocked)
const cooldown = await db.userPreference.findUnique({
  where: { userId_key: { userId, key: "..." } }
});

console.log(cooldown); // undefined - causes test failure

Solution

Create separate mock implementations for each query method, each checking the query parameters to return appropriate data.

Pattern 1: Separate Mocks for Each Method

import { vi, beforeEach, it, expect } from "vitest";

// Mock database client
const mockDb = {
  userPreference: {
    findFirst: vi.fn(),
    findUnique: vi.fn(),
  },
} as unknown as typeof db;

beforeEach(() => {
  vi.clearAllMocks();
});

it("should handle dual query pattern", async () => {
  const userId = "test-user-id";

  // Mock findFirst for enabled/disabled check
  mockDb.userPreference.findFirst = vi.fn().mockImplementation(({ where }) => {
    if (where.key === "email.notification.messaging") {
      return Promise.resolve({
        userId,
        key: "email.notification.messaging",
        value: "true",
      });
    }
    return Promise.resolve(null);
  });

  // Mock findUnique for composite key lookup (separate implementation)
  mockDb.userPreference.findUnique = vi.fn().mockImplementation(({ where }) => {
    if (where.userId_key?.key === "email.notification.messaging.lastSent") {
      return Promise.resolve({
        userId,
        key: "email.notification.messaging.lastSent",
        value: new Date().toISOString(),
      });
    }
    return Promise.resolve(null);
  });

  // Both queries now work correctly
  const result = await myFunction(mockDb, userId);

  expect(mockDb.userPreference.findFirst).toHaveBeenCalledWith({
    where: { userId, key: "email.notification.messaging" },
  });
  expect(mockDb.userPreference.findUnique).toHaveBeenCalledWith({
    where: {
      userId_key: {
        userId,
        key: "email.notification.messaging.lastSent",
      },
    },
  });
});

Pattern 2: Conditional Mock Based on Query Structure

it("should mock based on query structure", async () => {
  // Single mock that handles both query types
  mockDb.userPreference.findFirst = vi.fn().mockImplementation(({ where }) => {
    // Route based on query structure
    if (where.key === "specific.key") {
      return Promise.resolve({ userId, key: where.key, value: "data" });
    }
    return Promise.resolve(null);
  });

  mockDb.userPreference.findUnique = vi.fn().mockImplementation(({ where }) => {
    // Check for composite key pattern
    if (where.userId_key) {
      const key = where.userId_key.key;
      return Promise.resolve({ userId, key, value: "data" });
    }
    return Promise.resolve(null);
  });
});

Pattern 3: Using Vitest Mock Libraries

For complex scenarios, use specialized mocking libraries:

import { createPrismaMock } from 'prisma-mock-vitest';
import { beforeEach, expect, test } from 'vitest';

let client: PrismaClient;

beforeEach(async () => {
  client = await createPrismaMock();
});

test("automated mocking handles both queries", async () => {
  // Library automatically handles findFirst, findUnique, and other methods
  const result = await myFunction(client, userId);
  expect(result).toBeDefined();
});

Verification

Check Mock Calls: Verify both mocks are called with correct parameters

expect(mockDb.model.findFirst).toHaveBeenCalledTimes(1);
expect(mockDb.model.findUnique).toHaveBeenCalledTimes(1);

Test Isolation: Each mock should only respond to its specific query pattern

// findFirst should not respond to composite key queries
const result = await mockDb.model.findFirst({
  where: { userId_key: { userId, key } }
});
expect(result).toBeNull(); // Should not match

Return Value Validation: Ensure mocked data structure matches Prisma schema

const data = await mockDb.model.findUnique({ where: { id: "123" } });
expect(data).toHaveProperty("id");
expect(data).toHaveProperty("userId");

Example

Real-World Scenario: Email preference system checking both enabled state and cooldown.

// Code being tested
async function shouldSendEmail(db: DB, userId: string) {
  // Query 1: Check if notifications are enabled (findFirst)
  const preference = await db.userPreference.findFirst({
    where: { userId, key: "email.notification.messaging" }
  });

  if (preference?.value !== "true") {
    return { send: false };
  }

  // Query 2: Check cooldown timestamp (findUnique with composite key)
  const cooldown = await db.userPreference.findUnique({
    where: {
      userId_key: {
        userId,
        key: "email.notification.messaging.lastSent",
      },
    },
  });

  if (cooldown) {
    const lastSent = new Date(cooldown.value);
    // Check if within cooldown period...
  }

  return { send: true };
}

// Test with separate mocks
it("should check both enabled state and cooldown", async () => {
  const thirtyMinutesAgo = new Date(Date.now() - 30 * 60 * 1000);

  // Mock findFirst (enabled check)
  mockDb.userPreference.findFirst = vi.fn().mockImplementation(({ where }) => {
    if (where.key === "email.notification.messaging") {
      return Promise.resolve({
        userId: "test-user",
        key: "email.notification.messaging",
        value: "true", // Enabled
      });
    }
    return Promise.resolve(null);
  });

  // Mock findUnique (cooldown check)
  mockDb.userPreference.findUnique = vi.fn().mockImplementation(({ where }) => {
    if (where.userId_key?.key === "email.notification.messaging.lastSent") {
      return Promise.resolve({
        userId: "test-user",
        key: "email.notification.messaging.lastSent",
        value: thirtyMinutesAgo.toISOString(), // Recent send
      });
    }
    return Promise.resolve(null);
  });

  const result = await shouldSendEmail(mockDb, "test-user");

  expect(result.send).toBe(false); // Blocked by cooldown
  expect(mockDb.userPreference.findFirst).toHaveBeenCalledTimes(1);
  expect(mockDb.userPreference.findUnique).toHaveBeenCalledTimes(1);
});

Notes

Query Structure Matters: findFirst uses where: { field: value }, findUnique uses where: { uniqueConstraint: { field: value } } - check this in mocks
Mock Isolation: Each mock should ONLY respond to its specific query pattern
Clear All Mocks: Always call vi.clearAllMocks() in beforeEach to prevent test pollution
Type Safety: Use as unknown as typeof db to satisfy TypeScript when creating mock objects
Library Options: Consider prisma-mock-vitest, vitest-prisma-mock, or prisma-mock for complex scenarios instead of manual mocking
Debugging: Log where parameter in mockImplementation to see what queries are being made
Common Mistake: Mocking only findFirst when code also uses findUnique (or vice versa)

vitest-prisma-dual-query-mocking