英文:
Querying in One to many mappings in hibernate
问题
以下是您要翻译的部分：
我有一个书籍实体和一个图书馆实体（一个图书馆可以有多本书，但每本书只属于一个图书馆）。它们的定义如下：
public class Library
{
@Id
private Long id;
@OneToMany(mappedBy = "library")
private List<Book> books = new ArrayList<>();
}
和
public class Book
{
@Id
private Long id;
@ManyToOne
@JoinColumn(name="FK_Library", nullable=false)
private Library library;
}
我想要找到属于特定图书馆的所有书籍。一种方法是创建一个`LibraryRepository`，如下所示：
public interface LibraryRepository extends JpaRepository<Library, Long>{}
现在我有一个`findById(Long libraryId)`方法，它返回一个包含`List<Book>`的`Library`对象。
我想知道是否可能创建一个`BookRepository`，并声明一个名为`findByLibraryId(Long libraryId)`的方法，并让Hibernate通过外键`FK_Library`查询`Book`表，**而不执行任何连接操作**。
像这样：
public interface BookRepository extends JpaRepository<Book, Long> {
public List<Book> findByLibraryId(Long libraryId);
}
上述方法`findByLibraryId`确实执行连接操作。我不希望它执行连接操作，因为我只想查找由外键关系定义的所有具有`library_id`的行。
另外，第一种方法是否比第二种方法更可取？
英文:
Let's say I have a book entity and a library entity (one library can have multiple books, but each book belongs to only one library). They are defined as follows:
public class Library
{
@Id
private Long id;
@OneToMany(mappedBy = "library")
private List<Book> books = new ArrayList<>();
}
and
public class Book
{
@Id
private Long id;
@ManyToOne
@JoinColumn(name="FK_Library", nullable=false)
private Library library;
}
I want to find all books which belong to a particular library. One way is to create a LibraryRepository as follows:
public interface LibraryRepository extends JpaRepository<Library, Long>{}
Now I have findById(Long libraryId) method that returns a Library object with a List<Book> in it.
I was wondering if it is possible to create a BookRepository and declare a method named findByLibraryId(Long libraryId) and let hibernate query the Book table by the foreign key FK_Library, without performing any joins.
Something like this:
public interface BookRepository extends JpaRepository<Book, Long> {
public List<Book> findByLibraryId(Long libraryId);
}
The above method findByLibraryId does perform join. I do not want it to perform a join, since all I want is find all rows which have library_id defined by foreign key relationship.
Also is the first method more preferable then second one?
答案1
得分: 3
你可以使用JPQL的@Query来实现这个功能。
@Query("SELECT b FROM Book b WHERE b.library.id = :libraryId")
public List<Book> findByLibraryId(Long libraryId);
英文:
You can use @Query with JPQL to do that
@Query("SELECT b FROM Book b WHERE b.library.id = :libraryId")
public List<Book> findByLibraryId(Long libraryId);
答案2
得分: 2
你可以将@ManyToOne关系更改为懒加载。
@ManyToOne(fetch = FetchType.LAZY)
这是OneToMany的默认设置，这就是为什么在搜索Library时你看不到连接的原因。
英文:
You could change the @ManyToOne relationship to lazy loading.
@ManyToOne(fetch = FetchType.LAZY)
This is the default for OneToMany, thats why you see no join when you search the Library.
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