(Quick Reference)
belongsTo
目的
Purpose
Defines a "belongs to" relationship where the class specified by
belongsTo assumes ownership of the relationship. This has the effect of controlling how saves and deletes cascade. The exact behaviour depends on the type of relationship:
- Many-to-one/one-to-one: saves and deletes cascade from the owner to the dependant (the class with the
belongsTo).
- One-to-many: saves always cascade from the one side to the many side, but if the many side has
belongsTo, then deletes also cascade in that direction.
- Many-to-many: only saves cascade from the "owner" to the "dependant", not deletes.
使用例
Examples
class Book {
String title static belongsTo = [author: Author]
}In this example the
Book class specifies that it belongs to the
Author class, hence when an
Author instance is deleted so are all its associated
Book instances.
詳細
Description
The
belongsTo property abstracts the nature of the cascading behaviour in Hibernate. If you want one class to belong to another but not have a back reference, then you can specify a class or a list of classes as the value:
class Book {
static belongsTo = Author
}or:
class Book {
static belongsTo = [Author, Library]
}Back references, i.e. properties linking back to the owner, can be added in one of two ways:
class Book {
Author author static belongsTo = Author
}or:
class Book {
static belongsTo = [author: Author]
}In these examples, both techniques create an
Author property named
author. Also, the Map property can specify multiple properties and types if the class belongs to more than one owner.
The
belongsTo property is simple and means you don't have to worry about the Hibernate cascading strategies, but if you need more control over cascading you can use the
ORM DSL. This allows fine grained control of cascading updates and deletes.
