[play-framework] Mixins & JPA

[Leif Singer]

Hello!

I'm thinking about the best way to add generic capabilities to model classes. For example, I might have model classes names "User" and "Bookmark". Now, I want both of these to be "taggable". Later, I might want to add the "likable" capability to one or both of them.

I'm looking for a solution in Java that plays nice with JPA.

The best solution I've found so far is having the domain classes implement a "Taggable" / "Likable" interface and providing "AbstractTaggable" / "AbstractLikable" skeleton classes that contain default implementations the domain classes can use.

Still, in a domain class, I would need to manually create the interface methods and have them use the skeleton classes.

Does anyone have a best practice to share that would match this use case?

Thanks in advance
Leif

--
Leif Singer
http://leif.singr.org




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[Guillaume Bort]

It's difficult to apply the mixins concept to Java. Scala could help
you to do that. But I'm not sure about the limitations of using scala
mixins and JPA.

[Leif Singer]

Hello,

> It's difficult to apply the mixins concept to Java. Scala could help
> you to do that. But I'm not sure about the limitations of using scala
> mixins and JPA.

I wanted to avoid Scala because I don't know about the Eclipse support. So I was now looking further at a solution in Java, and seemingly what I need are polymorphic associations in some of my model classes. However, I couldn't find any documentation on how to create them using Hibernate, let alone JPA.

I'd be willing to accept a Hibernate dependency if that's what it takes, but right now I don't see any solution at all.

Basically what I need is this:

interface Taggable
addTag(Tag tag)
removeTag(Tag tag)

Document implements Taggable
@OneToMany
List<Tagging> taggings
addTag(Tag tag) {...}
removeTag(Tag tag) {...}

User implements Taggable
@OneToMany
List<Tagging> taggings
addTag(Tag tag) {...}
removeTag(Tag tag) {...}

Tagging
@ManyToOne
Tag tag
@ManyToOne
Taggable taggable

Tag
@OneToMany
List<Tagging> taggings

The Tagging.taggable @ManyToOne assoiciation is where I run into problems, as JPA seemingly can only create associations to @Entity classes.

Again, any pointers as to how to solve this would be greatly appreciated.

Thanks in advance
Leif

>> I'm thinking about the best way to add generic capabilities to model classes. For example, I might have model classes names "User" and "Bookmark". Now, I want both of these to be "taggable". Later, I might want to add the "likable" capability to one or both of them.
>>
>> I'm looking for a solution in Java that plays nice with JPA.
>>
>> The best solution I've found so far is having the domain classes implement a "Taggable" / "Likable" interface and providing "AbstractTaggable" / "AbstractLikable" skeleton classes that contain default implementations the domain classes can use.
>>
>> Still, in a domain class, I would need to manually create the interface methods and have them use the skeleton classes.
>>
>> Does anyone have a best practice to share that would match this use case?

[Guillaume Bort]

http://stackoverflow.com/questions/979200/hibernate-polymorphism

Perhaps it will help? Tell us.