GORM - Mapping inheritance

Added by graeme, last edited by graeme on Nov 09, 2006  (view change)

Mapping Inheritance

GORM uses table-per-heirarchy inheritance which essentially means the parent and all sub-classes share the same table:

class Content {
     String author
}
class BlogEntry extends Content {
    URL url
}
class Book extends Content {
    String ISBN
}
class PodCast extends Content {
    byte[] audioStream
}

The above then allows you to perform polymorphic queries:

def content = Content.findAll() // find all blog entries, books and pod casts
content = Content.findAllByAuthor('Joe Bloggs') // find all by author

def podCasts = PodCast.findAll() // find only pod casts

Technical note for those interested: Under the covers, only one table is used in the table-per-hierarchy model. A class column specifies the subclass and any fields in the subclasses are included on the same table. Subclass fields cannot be "required" because the underlying columns need to be nullable for the other subclasses. This doesn't however prevent you from using Validation constraints to ensure that subclass fields are "required".

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