The role of Tuple in PlayFramework

Question

Now I'm using play2.2.1, and I know Java, but I don't know Scala. And if you learn PlayFramework even just a little bit, you will know Scala is used in some extent in the framework. This is just a background about me. The question is why is Tuple2 used in below code although the number of passing values is one not plural.

This is the controller's code:

//Action for making a message
    public static Result add() {
        Form<Message> f = new Form(Message.class);
        List<Member> mems = Member.find.select("name").findList();
        List<Tuple2<String, String>> opts =
                            new ArrayList<Tuple2<String, String>>();
        for(Member mem: mems) {
            opts.add(new Tuple2(mem.name, mem.name));
        }
        return ok(add.render("fill-in form", f, opts));
    }

This is the Model's code:

@Entity
public class Member extends Model {

    @OneToMany(cascade = CascadeType.ALL)
    public List<Message> messages = new ArrayList<Message>();

    @Id
    public Long id;

    @Required(message = "This item is Required")
    public String name;

    @Email(message = "Please Fill in a message")
    public String mail;

    public String tel;

    public static Finder<Long, Member> find = new Finder<Long, Member>(Long.class, Member.class);

    @Override
    public String toString() {
        String ids = "{id:";
        for (Message m: messages) {
            ids += " " + m.id;
        }
        ids += "}";
        return ("[id:" + id + ", message:" + ids + ", name:" + name + ", mail:" + mail + ", tel:" + tel + "]");
    }

    public static Member findByName(String input) {
        return Member.find.where().eq("name", input).findList().get(0);
    }
}

This is the View's code:

@(msg:String, form1: Form[models.Message], opts: List[Tuple2[String, String]])

@main("Sample page") {
    <h1>ADD MESSAGE</h1>
    <p>@msg</p>
    @helper.form(action = routes.Application.create) {

    @(helper.select (
        field = form1("name"),
        options = opts
        ))

    @(helper.textarea (
        field = form1("message")
        ))

    <input type="submit">
    }
}

Again, why is Tuple2, which contains two String values, used despite one value passed from the controller to the view? If the information I showed here is not enough, please give me a comment.

Answer 1

The role of a tuple is to provide multiple values in a single instance. Perhaps it's a bit like an unnamed class? To summarise the comments and give a clear example, the structure of the <option> tag in a <select> structure is

<option value="internal-value">shown-value</option>

We use it often for foreign key situations - the foreign key value is in "internal-value" and the name or other attribute from the linked table is in the shown-value.

If you use the play helper @select to generate it, you should provide a Seq of tuple2s - i.e. two strings for each option. The first for the internal value, and the second for the displayed value. I use a defining but that is just one way to do it. For example:

@defining(for (a <- AppType.all()) yield (a.id.toString,a.name)) { appTypeList =>
    @select(
        apiForm("forApp"),
        appTypeList,
        'class -> "form-control",
        '_default -> "-- None Selected --",
        '_label -> "App Type"
    )
}

You can generate it yourself manually if you have some special needs or reason not to use the helper (or you can write your own helper :) ). For example:

<select name="forApiType" id="forApiType">
    @ApiType.all().map { a =>
      <option value="@a.id" @if(a.id==apiType.id) {selected}>
        @a.name
      </option>
    }
</select>