Check dynamic value for field in struct in go using testify

I have a user service that validates the user data and formats it and then calls Firebase service which creates a firebase user and return firebase id and then it pass that user data to repository layer. My user struct has an ID field is populated by uuid in user service before passing to repository layer. I am mocking the firebase service and repository layer using strecher/testify. But the test is failing since the ID is populated by service layer and I cannot pass the ID to the user data used by mock function.

user := model.User{
		ID:         "",
        FirebaseID: "",
		FirstName: "Test",
		LastName:  "User",
		FullName:  "Test User",
		Email:     "testuser@email.com"
		Password: "password",
	}

Service layer code

func (u userService) CreateUser(user model.User) error {
  err := validateFields(user)
  if err != nil {
	  return fmt.Errorf("userService CreateUser: %w", err)
  }

  user.FullName = user.FirstName + " " + user.LastName
  user.FirebaseID, err = u.authClient.CreateUser(user)
  if err != nil {
	  return fmt.Errorf("userService CreateUser: %w", err)
  }

  user.ID = uuid.NewString()
  err = u.userRepo.CreateUser(user)
  if err != nil {
	  return fmt.Errorf("userService CreateUser: %w", err)
  }

return nil

}

Test code

func TestCreateUser(t *testing.T) {
  mockFirebaseAuthClient := new(MockFirebaseAuthClient)
  mockPostgresRepo := new(MockPostgresRepo)
  userService := NewUserService(mockPostgresRepo, mockFirebaseAuthClient)

  t.Run("Valid data", func(t *testing.T) {
	  user := model.User{
		ID:         "",
		FirebaseID: "firebaseuniqueid",
		FirstName: "Test",
		LastName:  "User",
		FullName:  "Test User",
		Email:     "testuser@email.com",
		Password: "password",
	  }
	  mockFirebaseAuthClient.On("CreateUser", user).Return("firebaseuniqueid", nil)
	  mockPostgresRepo.On("CreateUser", user).Return(nil)
	  err := userService.CreateUser(user)
	  if err != nil {
		  t.Fatalf("Expectd: nil, got: %v", err)
	  }
})

Error while testing

mock: Unexpected Method Call
-----------------------------

CreateUser(model.User)
		0: model.User{ID:"f87fd2f3-5801-4359-a565-a4eb13a6de37", FirebaseID:"firebaseuniqueid", FirstName:"Test", LastName:"User", FullName:"Test User", Email:"testuser@email.com", Password:"password"}

The closest call I have is: 

CreateUser(model.User)
		0: model.User{ID:"", FirebaseID:"firebaseuniqueid", FirstName:"Test", LastName:"User", FullName:"Test User", Email:"testuser@email.com", Password:"password"}

Difference found in argument 0:

--- Expected
+++ Actual
@@ -1,3 +1,3 @@
 (model.User) {
- ID: (string) "",
+ ID: (string) (len=36) "f87fd2f3-5801-4359-a565-a4eb13a6de37",
  FirebaseID: (string) (len=16) "firebaseuniqueid",

Diff: 0: FAIL:  (model.User={f87fd2f3-5801-4359-a565-a4eb13a6de37 firebaseuniqueid Test User Test User testuser@email.com  password}) != (model.User={ firebaseuniqueid Test User Test User testuser@email.com  password}) [recovered]

Is there any way I could check the dynamically created uuid or ignore the values in the struct in the test?

if you don't want to consider mockFirebaseAuthClient.On("CreateUser", user).Return("firebaseuniqueid", nil) and mockPostgresRepo.On("CreateUser", user).Return(nil) and just want to mock that calls, then you can use mock.Anything as the argument in both the calls instead of user like this mockFirebaseAuthClient.On("CreateUser", mock.Anything).Return("firebaseuniqueid", nil) . So the arguments will not be considerd and the mock calls will return required value.

Regarding your question of

> Since uuid is not injected into the service layer, how can it be mocked? It is an imported package.

Like this, first, define an interface with the same method we want to mock

type uuidGen interface {
	String() string
}

Then, define a mock type in which we're going to define our method

type mockGen struct{}

Then, define the method on the type

func (u *mockGen) String() string {
	return "test"
}

Update CreateUser function to receive a uuidGen parameter that shares the method String() with uuid's package.

func (u userService) CreateUser(uuid uuidGen, user User) error {
	err := validateFields(user)
	if err != nil {
		return fmt.Errorf("userService CreateUser: %w", err)
	}
	user.FullName = user.FirstName + " " + user.LastName
	user.FirebaseID, err = u.authClient.CreateUser(user)
	if err != nil {
		return fmt.Errorf("authClient CreateUser: %w", err)
	}
	user.ID = uuid.String()
	err = u.userRepo.CreateUser(user)
	if err != nil {
		return fmt.Errorf("userService CreateUser: %w", err)
	}
	return nil
}

Now we can write the test like this, see how the 2 methods accept different types that implement the interface uuidGen and can call a method String()

func TestCreateUser(t *testing.T) {
	mockFirebaseAuthClient := new(MockFirebaseAuthClient)
	mockPostgresRepo := new(MockPostgresRepo)
	userService := NewUserService("test", "test")

	t.Run("Valid data", func(t *testing.T) {
			user := User{
					ID:         "",
					FirebaseID: "firebaseuniqueid",
					FirstName:  "Test",
					LastName:   "User",
					FullName:   "Test User",
					Email:      "testuser@email.com",
					Password:   "password",
			}
			mockFirebaseAuthClient.On("CreateUser", user).Return("firebaseuniqueid", nil)
			mockPostgresRepo.On("CreateUser", user).Return(nil)
			gen := new(mockGen)
			err := userService.CreateUser(gen, user)
			if err != nil {
					t.Fatalf("Expectd: nil, got: %v", err)
			}
			realUUID := uuid.New()
			err = userService.CreateUser(realUUID, user)
			if err != nil {
					t.Fatalf("Expectd: nil, got: %v", err)
			}
			t.Log("Mock UUID:", gen.String())  // prints test
			t.Log("Real UUID UUID:", realUUID.String()) // prints a UUID
	})
}

Run it with go test -v to see the output of t.Log(...)