Learn Java for Web Development

348 CHAPTER 7: Rapid Web Development with Grails

In Listing 7-31 there is a unidirectional one-to-many. Grails will, by default, map this kind of relationship
with a join table. Grails will automatically inject a property of type java.util.Set into the domain class
based on the hasMany setting. Grails supports many-to-many relationships by defining a hasMany on
both sides of the relationship and having a belongsTo on the owned side of the relationship.

You will create an application with a many-to-many domain relationship between Author and Book.
You can find the source code for this application in the books project in Chapter 7 of the source
code archive that you can download from the Apress website. Listing 7-32 illustrates the Book class.

Listing 7-32. Creating a Domain Relationship Between Book and Author

1.package books
2.
3.class Book {

4. static belongsTo = Author


5. static hasMany = [authors:Author]


6. String title
   7.Long isbn
   8.String publisher
   9.static constraints = {
   10.title(blank:false)
   11.}


7. 
   

   13.String toString() {
   14.title
   15.}
   16.}

   
   

   Line 4: This line informs Grails that the Book class belongs to its owning Author.

   Line 5: This line inform Grails that the Book class has many instances of Author.

Listing 7-33 illustrates the Author class.

Listing 7-33. Author class.

1.package books
2.
3.class Author {
4.

5. static hasMany = [books:Book]


6. String firstName
   7.String lastName
   8.static constraints = {
   9.firstName(blank:false)
   10.lastName(blank:false)
   11.}
   12.String toString() {
   13."$lastName, $firstName"
   14.}
   15.}

   Line 5: This line tells Grails that an Author has many instances of Book.