broken relations when loading from fixture / yaml in play framework - jpa

Question

I have found a problem with the play framework. I could also reproduce it, so here I will show a simplified reproduction szenario.
When starting the play application, I want to read sample data from a yaml file. Therefore, i use the class Fixtures. In the yaml file I have prepared a data structure of objects that stand in relation to each other.
The model classes of the data structure look like this:
#Entity
public class Album extends Model{
public String name;
#ManyToOne
public Artist artist;
}
#Entity
public class Artist extends Model{
public String name;
#OneToMany(mappedBy="artist")
public List<Album> albums = new ArrayList<Album>();
}
The Job that i use to load the yaml file and control the result looks like this:
#OnApplicationStart
public class Bootstrap extends Job {
#Override
public void doJob(){
Fixtures.deleteAllModels();
Fixtures.loadModels("sample.yml");
List<Artist> artists = Artist.findAll();
for (Artist artist : artists) {
play.Logger.info(artist.name + " has " + artist.albums.size() + " albums");
}
}
}
if i use the following structure in my yml file, then it works:
Artist(b1):
name: Nirvana
Artist(b2):
name: ACDC
Album(a1):
name: Back in Black
artist: b2
Album(a2):
name: Highway to Hell
artist: b2
Album(a3):
name: Nevermind
artist: b1
Album(a4):
name: Bleach
artist: b1
But if i do it like this, then it will NOT work:
Album(a1):
name: Back in Black
Album(a2):
name: Highway to Hell
Album(a3):
name: Nevermind
Album(a4):
name: Bleach
Artist(b1):
name: Nirvana
albums: [a3,a4]
Artist(b2):
name: ACDC
albums: [a1,a2]
However, the documentation right here tells us, that the second way should work.
Did I make a mistake in my example code, or is this really a problem with the play framework or JPA?

Answer 1

No, according documentation your second try should not work. Problem lies in concept of relationship owner. Only owner side (one referenced by mappedby) is consulted when bidirectional relationship is persisted.
In your case
Artist(b1):
name: Nirvana
albums: [a3,a4]
Operates to following list, which is not owner of relationship:
//owner of this relationship if attribute artist in Album entity.
#OneToMany(mappedBy="artist")
public List<Album> albums = new ArrayList<Album>();
Your first try uses artist field in Album. It works, because artist is owner of bidirectional relationship between Album and Artist. Because of same reason also example in documentation that you linked works as well.