Play框架-删除对象后在会话中找不到用户ID

Question

I have a table listing registered activities with the possibility to delete each of them. The table looks like this:

<table class="table-striped table-condensed table-hover">
    <tr>
        <th>Date</th>
        <th>Activity</th>
        <th>Steps</th>
        <th></th>
    </tr>
    @for(a <- activities) {
    @helper.form(routes.DashboardController.deleteUA(a.id)) {
        <tr>
            <td style="width:30%">@a.date.format("dd MMM")</td>
            <td style="width:30%">@a.activity.name</td>
            <td style="width:30%">@a.steps</td>
            <td>
            <button type="submit" style="background-color:#FF6666"><i class="fa fa-trash-o"></i></button>
            </td>
            </tr>
    }}
</table>

The method to delete the UserActivity object looks like this:

public static Result deleteUA(Long id) {
    UserActivity.find.ref(id).delete();
    return dashboard();
}

When the object is deleted the method will render the page again with the dashboard() method:

@Security.Authenticated(Secured.class)
public static Result dashboard() {
    User user = User.find.byId(request().username());
    return ok(dashboard.render(user, Tips.all(), 0.0 , getGoals(user), updateLeaderboards(), getRecentUA()));
}

When I submit the form and call on the deleteUA() the object is deleted as expected, but when I'm going to render the page with dashboard() I get this error:

[NullPointerException: The id is null]

from this line:

User user = User.find.byId(request().username());

If I go back to the dashboard by calling on the url (calling on the dashboard() method that way) everything seems fine and the deleted activity is gone from the list.

I can't understand why the deletion of an UserActivity object has anything to do with the finding of the user id in the session though. Any ideas?

Update! UserActivity looks like this:

package models;

    import java.util.*;
    import play.db.ebean.*;
    import javax.persistence.*;

    @Entity
    public class UserActivity extends Model {

        @Id
        public Long id;
        @ManyToOne 
        public User belongsTo;
        @ManyToOne  
        public Activity activity;
        public int intensity;
        public double steps;
        public Date date;

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

Answer 1

How about trying to use the redirect(...) method instead of just calling the controller's action. I would assume that your routes file has some entry like this

GET /dashboard controllers.Application.dashboard()

Then in your deleteUA method you can write

public static Result deleteUA(Long id) {
    UserActivity.find.ref(id).delete();
    return redirect(controllers.Application.dashboard());
}