如何从Lift中检索多对多关系的数据

Question

假设我有3个表：Book，Author和BookAuthor。

Book has id, title
Author has id, name
BookAuthor has id, book_id, author_id

我想找到一本书的所有作者。任何人都可以用Lifts mapper语法指导我如何做到这一点吗？

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

这是本书的映射器：

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]

特质IdPk将负责Book的ID。然后为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]

然后为作者，一个简单的映射器：

class Author extends LongKeyedMapper[Author] with IdPk {
  def getSingleton = Author
  object name extends MappedString(this, 200)
}
object Author extends Author with LongKeyedMetaMapper[Author]

然后调用以查找书籍的所有作者（此处为myBook）：

myBook.BookAuthor.map(x => x.Author.name)

如果您想要制作更复杂的加入请求而无需过滤Scala中的所有内容，则可以始终使用DB，并始终可以找到有关mapper here的更多信息。