GORM Criteria Query for One-to-Many

In Grails, GORM has some very nice features, not the least of which are Dynamic Finders. It's quite easy to sometimes overlook a really powerful and elegant alternative to dynamic finders in the form of Criteria. In this article I will cover one specific case in which there exists no dynamic finder for a specific use case, but for which a simple, elegant Criteria will do.


Consider the following one-to-many relationship in which One Author has many Books.

   1:class Book{
   2:    String title
   3:    static belongsTo = Author
   4:}
   5:
   6:class Author {
   7:    String name
   8:    static hasMany = [books: Book]
   9:}

The problem: Assuming we are holding a Book object, how do we find the associated Author for that book ? Here's how:

   1:def book = Book.findByTitle('Definitive Guide to Grails')
   2:
   3:def criteria = Author.createCriteria()
   4:
   5:def author = criteria {
   6:   books{
   7:      eq('id',book.id)
   8:   }
   9:}

The often overlooked "trick" to the above criteria, is the fact that we can use the "books" property on Author in our Criteria to match against (line 6 above).

References:

• The GORM section of the Grails User Guide