How to properly test a handler that calls another function inside of it - unit-testing

Question

I am looking to test the PostUser function that looks something like this (error handling omitted for simplicity):
func PostUser(env *Env, w http.ResponseWriter, req *http.Request) error {
decoder := json.NewDecoder(req.Body)
decoder.Decode(&user)
if len(user.Username) < 2 || len(user.Username) > 30 {
return StatusError{400, errors.New("usernames need to be more than 2 characters and less than 30 characters")}
}
emailRe := regexp.MustCompile(`^[a-z0-9._%+\-]+#[a-z0-9.\-]+\.[a-z]{2,4}$`)
if !emailRe.MatchString(user.Email) {
return StatusError{400, errors.New("invalid email address")}
}
if len(user.Password) < 8 {
return StatusError{400, errors.New("passwords need to be more at least 8 characters")}
}
hashedPassword,_ := bcrypt.GenerateFromPassword([]byte(user.Password), 12)
env.DB.InsertUser(user.Username, hashedPassword, user.Email) // need to mock this out
userData,_ := json.Marshal(user)
defer req.Body.Close()
w.Write(userData)
return nil
}
My env.go file looks like this:
type Env struct {
DB *db.DB
}
My db.go file looks like this:
type DB struct {
Session *mgo.Session
}
How do I mock the InsertUser call by my DB struct, so that I can unit test the PostUser?

Answer 1

To use mocks for testing you need to create an interface that your mock can implement. Naturally, the struct you're replacing the mock with also needs to implement all the methods of the interface so that they are freely interchangeable.
For example, you could have an interface:
type DBInterface interface {
InsertUser(string, string, string)
//all other methods on the DB struct here
}
Then your DB struct already implements all the methods of the interface. From there you can create a mock struct that also implements the interface.
type DBMock struct {}
func (dbm *DBMock) InsertUser(username, password, email string) {
//whatever mock functionality you want here
return
}
//all other methods also implemented.
Then you can alter env to have a pointer to a DBInterface instead of a DB. When you setup the env to be passed into the handler, in the production version use the DB struct and in testing use the DBMock struct.