How to retrieve data from many to many relationship with Lift - lift

Question

Let say I have 3 tables: Book, Author and BookAuthor.
Book has id, title
Author has id, name
BookAuthor has id, book_id, author_id
I want to find all authors of a book. Can anyone guide me how to do this with Lifts mapper syntax?

Answer 1

class Book extends LongKeyedMapper[Book]
with IdPK
with ManyToMany {
def getSingleton = Book
object title extends MappedString(this, 255)
object authors extends MappedManyToMany(
BookAuthors, BookAuthors.book, BookAuthors.author, Author)
}
object Book extends Book with LongKeyedMetaMapper[Book]
class Author extends LongKeyedMapper[Author]
with CreatedUpdated with IdPK
with ManyToMany {
def getSingleton = Author
object firstName extends MappedString(this, 255)
object lastName extends MappedText(this)
object email extends MappedEmail(this, 150)
object books extends MappedManyToMany( BookAuthors,
BookAuthors.author, BookAuthors.book, Book)
}
object Author extends Author with LongKeyedMetaMapper[Author]
val warandpeace = Book.create.title("War and Peace").saveMe
val fred = Author.create.firstName("Fred").saveMe
fred.books += warandpeace
fred.saveMe
val bob = Author.create.firstName("Bob").saveMe
bob.books += warandpeace
bob.saveMe
// then to find all the authors of the book:
val authors = warandpeace.authors

Answer 2

Here is the mapper for the book:
class Book extends LongKeyedMapper[Book] with IdPk with OneToMany[Long, Book] {
def getSingleton = Book
object title extends MappedString(this, 200)
object BookAuthor extends MappedOneToMany(BookAuthor, BookAuthor.id)
}
object Book extends Book with LongKeyedMetaMapper[Book]
The trait IdPk will take care of the id of Book. Then for BookAuthor:
class BookAuthor extends LongKeyedMapper[BookAuthor] with IdPk with OneToOne[Long, BookAuthor] {
def getSingleton = BookAuthor
object Author extends MappedOneToOne(Author, Author.id)
}
object BookAuthor extends BookAuthor with LongKeyedMetaMapper[BookAuthor]
Then for Author, a simple mapper:
class Author extends LongKeyedMapper[Author] with IdPk {
def getSingleton = Author
object name extends MappedString(this, 200)
}
object Author extends Author with LongKeyedMetaMapper[Author]
Then the call to find all authors of a book (here myBook ):
myBook.BookAuthor.map(x => x.Author.name)
If you want to make more complicated join requests without having to filter everything in Scala, you can always use DB and you can always find more info about mapper here.