# belongsTo

## 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 you have:

- 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.

## 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 the `Book` instances associated with the `Author`

## Description

The `belongsTo` property serves to abstract 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 the above examples, both techniques create a property called `author` of type `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 take advantage of the ORM DSL. This allows fine grained control of cascading updates and deletes.