Objectify doesn't always return results

Question

I am using Objectify to store data on Google App Engine's datastore. I have been trying to implement a one-to-many relationship between two classes, but by storing a list of parameterised keys. The method below works perfectly some of the time, but returns an empty array other times - does anyone know why this may be?

It will either return the correct list of CourseYears, or

{
 "items": [
 ]
}

Here is the method:

@ApiMethod(name = "getCourseYears") @ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
public ArrayList<CourseYear> getCourseYears(@Named("name") String name){

    Course course = ofy().load().type(Course.class).filter("name", name).first().now();
    System.out.println(course.getName());

    ArrayList<CourseYear> courseYears = new ArrayList<CourseYear>();
    for(Key<CourseYear> courseYearKey: course.getCourseYears()){
        courseYears.add(ofy().load().type(CourseYear.class).id(courseYearKey.getId()).now());
    }

    return courseYears;
}

The Course class which stores many CourseYear keys

@Entity
public class Course {
@Id
@Index
private Long courseId;

private String code;

@Index
private String name;

@ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
public List<Key<CourseYear>> getCourseYears() {
    return courseYears;
}

@ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
public void setCourseYears(List<Key<CourseYear>> courseYears) {
    this.courseYears = courseYears;
} 

@ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
     public void addCourseYear(Key<CourseYear> courseYearRef){
    courseYears.add(courseYearRef);
}
@Load
@ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
List<Key<CourseYear>> courseYears = new ArrayList<Key<CourseYear>>();

...

}

I am debugging this on the debug server using the API explorer. I have found that it will generally work at the start for a few times but if I leave and return to the API and try and run it again, it will not start working again after that.

Does anyone have any idea what might be going wrong?

Many thanks.

konqi konqi · Accepted Answer · 2015-04-24T15:08:54

You might want to reduce the amount of queries you send to the datastore. Try something like this:

Course course = ofy().load().type(Course.class).filter("name", name).first().now();

ArrayList<CourseYear> courseYears = new ArrayList<CourseYear>();
List<Long> courseIds = new List<>();
for(Key<CourseYear> courseYearKey: course.getCourseYears()){
    courseIds.add(courseYearKey.getId());
}
Map<Long, Course> courses = ofy().load().type(CourseYear.class).ids(courseIds).list();
// add all courses from map to you courseYears list

I also strongly recommend a change in your data structure / entities:

In your CourseYears add a property Ref<Course> courseRef with the parent Course and make it indexed (@Index). Then query by

ofy().load().type(CourseYear.class).filter("courseRef", yourCourseRef).list();

This way you'll only require a single query.

Objectify doesn't always return results

2 Answers