Play Framework 2: Best way of validating individual model fields separately

Question

For this example, lets assume the user would like to update just the first name of his online profile.

Form:

<form data-ng-submit="updateFirstName()">
  <label for="firstName">First name<label>
  <input type="text" name="title" data-ng-model="firstName">
  <button type="submit">Update first name</button>
</form>

Controller:

public class UsersController {
  public static Result updateFirstName() {
    Form<User> filledForm = Form.form(User.class).bindFromRequest();

    // TODO: Validate firstName

    // if hasErrors() return bad request with errors as json

    // else save and return ok()
  }
}

Model:

@Entity
public class User extends Model {
  @Id
  public Long id;
  @Constraints.Required
  public String firstName;
  @Constraints.Required
  public String lastName;
}

How would one validate just one field at a time against the models constraints and return any resulting error messages back as json? This is quite a simple example, the real thing will have many fields (some very complex) together with a form for each.

Answer 1

Play's in-built validation annotations conform to the Java bean validation specification (JSR-303). As a result, you can use the validation groups feature documented in the spec:

Model

@Entity
public class User extends Model {

  // Use this interface to mark out the subset of validation rules to run when updating a user's first name
  public interface FirstNameStep {}

  @Id
  public Long id;

  @Required(groups = FirstNameStep.class)
  public String firstName;

  // This isn't required when updating a user's first name
  @Required
  public String lastName;
}

Controller

public class UsersController {

  public static Result updateFirstName() {

    // Only run the validation rules that need to hold when updating a user's first name
    Form<User> filledForm = Form.form(User.class, User.FirstNameStep.class).bindFromRequest();

    if (form.hasErrors()) {
      // return bad request with errors as json
    }

    // else save and return ok()
  }
}

Validation groups are intended for your situation, where you have the same model object backing different forms, and you want to enforce different validation rules for the forms.