===============================domain/user_dao.go
package domain

import (
    "fmt"
    "log"

    "github.com/jkrovnl/golearn2/utils"
)

var (
    users = map[int64]*User{
        123: {Id: 321, Fname: "Janis", Lname: "Lange", Email: "[email protected]"},
    }
    UserDao userDaoInterface
)

func init() {
    UserDao = &userDao{}
}

type userDaoInterface interface {
    GetUser(int64) (*User, *utils.ApplicationError) //ja int32 tad init nestradas jo nav pareizi parametri
}

type userDao struct{}

func (u *userDao) GetUser(userId int64) (*User, *utils.ApplicationError) {
    log.Println("We are accesing database.")
    if user := users[userId]; user != nil {
        return user, nil
    }
    return nil, &utils.ApplicationError{
        Message:    fmt.Sprintf("user %v was not found", userId),
        StatusCode: 404,
        Code:       "not_found",
    }
}
===============================services/user_service.go
package services

import (
    "github.com/jkrovnl/golearn2/domain"
    "github.com/jkrovnl/golearn2/utils"
)

type usersService struct{}

var UsersService usersService

func (u *usersService) GetUser(userId int64) (*domain.User, *utils.ApplicationError) {
    return domain.UserDao.GetUser(userId)
}
===============================services/user_service.go
package services

import (
    "net/http"
    "testing"

    "github.com/jkrovnl/golearn2/domain"
    "github.com/jkrovnl/golearn2/utils"

    "github.com/stretchr/testify/assert"
)

var (
    userDaoMock     usersDaoMock
    getUserFunction func(userId int64) (*domain.User, *utils.ApplicationError)
)

func init() {
    domain.UserDao = &usersDaoMock{}
}

type usersDaoMock struct{}

func (n *usersDaoMock) GetUser(userId int64) (*domain.User, *utils.ApplicationError) {
    return getUserFunction(userId)
}

func TestGetUserNotFoundInDatabase(t *testing.T) {
    getUserFunction = func(userId int64) (*domain.User, *utils.ApplicationError) {
        return nil, &utils.ApplicationError{
            StatusCode: http.StatusNotFound,
            Message:    "user 0 was not found",
        }
    }
    user, err := UsersService.GetUser(0)
    assert.Nil(t, user)
    assert.NotNil(t, err)
    assert.EqualValues(t, http.StatusNotFound, err.StatusCode)
    assert.EqualValues(t, "user 0 was not found", err.Message)
}

func TestGetUserNoError(t *testing.T) {
    getUserFunction = func(userId int64) (*domain.User, *utils.ApplicationError) {
        return &domain.User{
            Id: 321,
        }, nil
    }
    user, err := UsersService.GetUser(123)
    assert.Nil(t, err)
    assert.NotNil(t, user)
    assert.EqualValues(t, 321, user.Id)
}