(Beginner) How could I retrive joined entities with Objectify ?

[Sébastien Vermeille]

Hi devs,

I'm beginning with Objectify I'm used of Relational Database so I encounter some pain with gae storage.

I made an entity called "Project" :

@Id
Long id

@index

String project

@Index

@Load

List<Ref<User>> developpers = new ArrayList<Ref<User>>();

Then I would like to do a method in my dao to retrive all project where a specified user is affected to :

public List<Project> getByUsername(String username) {

User user = DAOFactory.getUserDAO().getByUsername(username);

if(user != null) {

                        // This query is wrong I know. But I dont see how I can do ... in sql I would say "join user_project ... where user_project.id_user = ..." but I can't ^^

List<Project> founds = ofy().load().type(Project.class).filter(user, " in developpers").list();

return founds;

}

return null;

}

I didn't find something talking about it on the documentation does someone have any tips about it ? Thank you in advance :)

Sebastien Vermeille

[Alejandro Gonzalez]

As far as i know you cannot filter in Ref<?> properties. Instead i would do something like:

@Entity

public class Developper {

  @Id Long id;

  List<Long> projects;

}

@Entity

public class Project {

  @Id Long id;

  String name;

}

public List<Project> getDevelopperProjects( Long developperId ){

  

  Developper dev = ofy().load().type(Developper.class).id( developperId ).get();

  return new ArrayList<Project>( ofy().load().keys( dev.projects ).values() );

}

public void addDevelopperToProject( Long projectId, Long developperId ){

   Developper dev = ofy().load().type(Developper.class).id( developperId ).get();

   dev.projects.add( projectId );

   ofy().save().entity( dev ).now();

[Jeff Schnitzer]

This is not quite correct.  Ref<?> properties are simply keys in the datastore.  So a List<Ref<?>> is a List<Key>. You can't filter by properties on the referenced thing (that would be a join), but you can certainly filter by the key value itself.

The standard form for querying on a list property will work:

List<Project> founds = ofy().load().type(Project.class).filter(user, "developpers").list();

The 'user' value can be anything that represents a key - a Ref<?>, a Key<?>, a native datastore Key, or even a POJO entity of the appropriate type (as long as its id is set properly).

Jeff

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