test_user.go

type TestSetter struct {
	cmd *redis.StatusCmd
}

func (t *TestSetter) Set(k string, v interface{}, d time.Duration) *redis.StatusCmd {
	return t.cmd
}

// Unit - make it fail
func Test_setUser(t *testing.T) {
	expectedErr := errors.New("some random error")
	ts := &TestSetter{
		cmd: redis.NewStatusResult("", expectedErr),
	}
	err := setUser(ts, nil)
	if err != expectedErr {
		t.Error("unexpected error: %s", err)	
	}
}

// End to End - connect to a real redis server
func TestSetUser(t *testing.T) {
	rc := redis.NewClient(...)
	db := &UserDB{rc}
	u := &User{...}
	err := db.SetUser(u)
	if err != nil {
		t.Error("unexpected error: %s", err)	
	}
}

user.go

type User struct {
	Id string `json:"id"`
	Name string `json:"name"`
}

type UserDB struct {
	RedisClient *redis.Client	
}

func (db *UserDB) SetUser(u *User) error {
	return setUser(db.RedisClient, u)
}

type setter interface {
	Set(string, interface{}, time.Duration) *redis.StatusCmd
}

func setUser(db setter, u *User) error {
	// you probs wanna do some encoding here to bytes or json or whatever
	return db.Set(fmt.Sprintf("user.%s", u.Id), u, 0).Err()
}