The Grails Framework 3.1.4

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(Quick Reference) # addTo* ## Purpose Adds a domain class relationship for one-to-many or many-to-many relationship, where the relationship is indicated by the property used as the suffix to the method. ## Examples
    def fictBook = new Book(title: "IT")
    def nonFictBook = new Book(title: "On Writing: A Memoir of the Craft")
    def a = new Author(name: "Stephen King")
                 .addToFiction(fictBook)
                 .addToNonFiction(nonFictBook)
                 .save()
## Description The `addTo*` method is a dynamic method that uses an association to automatically add instances to the association. It also automatically configures the bi-directional relationship so that both sides are set. Consider these domain classes which are used in the example above:

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

    class Book {    String title    static belongsTo = [author: Author]
    }
The example creates a new fiction book and a new non-fiction book, and an author. `addToFiction` is a dynamic method that works because the `Fiction` part corresponds to the `fiction` collection that results from the `hasMany` declaration, and likewise for `NonFiction` and `nonFiction`. After calling `save()` on the `Author`, the associated `Book` instances are transitively saved too, even though we didn't call `save()` on either of them. In addition, calling addTo* initializes the associated collections, and sets the back-reference of the `author` property in each `Book`. There's also a more compact version of the method that accepts a `Map` instead of a domain class instance:
    def a = new Author(name: "Stephen King")
                 .addToFiction(title: "IT")
                 .addToNonFiction(title: "On Writing: A Memoir of the Craft")
                 .save()
This works because GORM knows the type that is being added and can use the standard `Map` constructor to create the instances for you. | Quick Reference (hide)