Managing Relationships

As of 0.5 this syntax is deprecated! See next section for migration path.

Since 0.3, Grails provides quite a few methods to ease relationship management. Firstly if you have a one-to-many relationship such as the Author-Book relationship defined below:

class Author {
  String name
  static hasMany = [books:Book]
}
class Book {
  String title
  Author author
  static belongsTo = Author
}

You can use the dynamic addBook method on author to ease relationship management:

def a = new Author(name:"Stephen King")
             .addBook(new Book(title:"IT"))
             .addBook(new Book(title:"The Stand"))
             .save()

The addBook method is available at runtime and uses the class name of the Book to automatically establish the relationship. If it is bidirectional it will also make sure that the author property of the Book instance is set.

If you have more than one one-to-many relationship defined for the same class:

class Author {
  String name
  static hasMany = [fiction:Book, nonFiction:Book]
}

You can use the add method which takes a to argument which defines which collection to add the book to:

def a = new Author(name:"Stephen King")
             .add(to:"fiction",
                   new Book(title:"IT"))
             .add(to:"nonFiction",
                   new Book(title:"On Writing: A Memoir of the Craft"))
             .save()

class Author {
  String name
  static hasMany = [books:Book]
}
class Book {
  String title
  Author author
  static belongsTo = Author
}

def a = new Author(name:"Stephen King")
             .addToBooks(title:"IT")
             .addToBooks(title:"The Stand")
             .save()

author.removeFromBooks(myBook)