4. Business rules

A title can't be blank or longer than 200 characters, and completing a task must not erase its title. These are rules, and rules live in the service. In this chapter you write them and learn how a rule tells the client what went wrong.

One rule, one function

Both creating and renaming a task need the same title checks. Add them to the end of preas/task/service.go:

const maxTitle = 200

func checkTitle(title string) error {
	switch {
	case title == "":
		return core.NewError(http.StatusBadRequest, "title can't be blank")
	case utf8.RuneCountInString(title) > maxTitle:
		return core.NewError(http.StatusBadRequest, "title is too long: 200 characters at most")
	}
	return nil
}

core.NewError creates an error that carries an HTTP status. The handler doesn't need to know which status fits; the error already says it.

utf8.RuneCountInString counts characters, not bytes, so "café" counts as 4. Add "unicode/utf8" to the imports.

Use it when creating

func (s *Service) Create(ctx context.Context, req dtos.CreateRequest) (Task, error) {
	req.Title = strings.TrimSpace(req.Title)
	if req.Title == "" { 
		return Task{}, core.NewError(http.StatusBadRequest, "title can't be blank") 
	if err := checkTitle(req.Title); err != nil { 
		return Task{}, err 
	}
	return s.Repo.Create(ctx, req)
}

Updates change only what was sent

A client completing a task sends just {"done": true}. The update must change done and leave the title alone. This is why the fields of UpdateRequest are pointers:

// UpdateRequest is the body of PATCH /task/:id. Only the fields you send
// change: nil keeps the column as it is.
type UpdateRequest struct {
	Title *string `json:"title"`
	Done  *bool   `json:"done"`
}

A field the client didn't send stays nil, and the repository's Update skips it. A field it sent, even false, is a pointer to that value, and gets written. With plain bool and string, there would be no way to tell "set it to false" from "not sent".

Check the title only when it was sent:

func (s *Service) Update(ctx context.Context, id string, req dtos.UpdateRequest) (Task, error) {
	if req.Title != nil {
		title := strings.TrimSpace(*req.Title)
		if err := checkTitle(title); err != nil {
			return Task{}, err
		}
		req.Title = &title
	}
	return s.Repo.Update(ctx, id, req)
}

Try it

Complete the task you created, using its id:

curl -X PATCH localhost:8000/task/4caff630-444a-4a86-bb31-3b5e963cfb32 \
  -H 'Content-Type: application/json' -d '{"done": true}'

{
    "code": 1,
    "data": {
        "id": "4caff630-444a-4a86-bb31-3b5e963cfb32",
        "title": "Buy coffee",
        "done": true
    }
}

The title survived. Now break a rule:

curl -X POST localhost:8000/task -H 'Content-Type: application/json' -d '{"title": "   "}'

{
    "text": "title can't be blank",
    "code": 2
}

The status is 400, and code: 2 marks a warning: the client asked for something it can fix.

Some errors come from the database itself. An id that isn't a valid UUID replies 400 invalid input, and an id that doesn't exist replies 404 not found. Errors & responses lists them all.

What you learned

• Rules belong in the service, written once and shared.
• core.NewError gives an error its HTTP status, and the handler just passes it on.
• Pointer fields let an update tell "not sent" from a zero value.

Next: a second preá →