JPA polymorphic oneToMany

Question

I couldn\'t figure out how to cleanly do a tag cloud with JPA where each db entity can have many tags.

E.g

Post can have 0 or more Tags User can have 0 or mo

Answer 1

Is there a better way in JPA than having to make all the entities subclass something like Taggable abstract class?

Let's forget the example :) JPA does support polymorphic associations but the target classes have to be part of an inheritance hierarchy. And here are some more rules of thumb about inheritance strategies:

• SINGLE_TABLE:
  • All the classes in a hierarchy are mapped to a single table
  • This strategy provides good support polymorphic relationships between entities and queries that cover the entire class hierarchy.
  • May contain null fields for some subclass data
• TABLE_PER_CLASS:
  • Each class in a hierarchy mapped to a separate table and hence, provides poor support for polymorphic relationships
  • requires SQL union or separate SQL queries for each subclass
• JOINED
  • no null fields => compact data
  • This provides good support for polymorphic relationships, but requires one or more join operations – may result in poor performance

In short, if your subclasses declare relatively few properties, prefer the SINGLE_TABLE strategy. If not, use a JOINED strategy unless you have a deep hierarchy (in which case the cost of joins may become more expensive than unions and then TABLE_PER_CLASS would be "less worse").

References

• JPA 1.0 Specification
  • Section 2.1.9 "Inheritance"
  • Section 2.1.10 "2.1.10 Inheritance Mapping Strategies"