[英]Is it possible to persist a List<E> with hibernate annotations, where E is an interface?
這似乎應該很簡單,但是我很難提出一個優雅的解決方案。
讓我們以一個基本文件系統為例,該系統由兩個帶有hibernate注釋的類File和Folder組成 。
我們將把這兩個類的通用屬性抽象到FileSystemObject接口中:
public interface FileSystemObject {
public String getName();
public URI getWhere();
}
和兩個類:
檔案 :
@Entity
@Table(name = "FILE")
public class File implements FileSystemObject, Serializable {
private static final long serialVersionUID = 1L;
@Id
@Column(name = "FILE_ID")
@GeneratedValue(strategy = GenerationType.AUTO)
private long id;
@Column(name = "NAME", nullable = false)
@Index(name = "FILE_NAME", columnNames={"NAME"})
private String name;
@Column(nullable = false)
private URI where;
public File() {}
public File(String name, URI where) {
this.name = name;
this.where = where;
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public URI getWhere() {
return where;
}
public void setWhere(URI where) {
this.where = where;
}
}
資料夾 :
@Entity
@Table(name = "FOLDER")
public class Folder implements FileSystemObject, Serializable {
private static final long serialVersionUID = 2L;
@Id
@Column(name = "FOLDER_ID")
@GeneratedValue(strategy = GenerationType.AUTO)
private long id;
@Column(name = "NAME", nullable = false)
@Index(name = "FOLDER_NAME", columnNames={"NAME"})
private String name;
@Column(nullable = false)
private URI where;
@CollectionOfElements
@JoinTable(name = "FOLDER_CONTENTS",
joinColumns = @JoinColumn(name = "FOLDER_ID"))
private List<FileSystemObject> contents;
public Folder() {}
public Folder(String name, URI where, List<FileSystemObject> contents) {
this.name = name;
this.where = where;
this.contents = contents;
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public URI getWhere() {
return where;
}
public void setWhere(URI where) {
this.where = where;
}
public List<FileSystemObject> getContents() {
return contents;
}
public void setContents(List<FileSystemObject> contents) {
this.contents = contents;
}
}
從邏輯上講, Folder的內容可以是File或Folder ,因此目錄List的類型為FileSystemObject是有意義的。
現在,假設當我嘗試生成一些模式時,所有內容都在pom.xml中正確設置了:
mvn hibernate3:hbm2ddl
它引發以下錯誤:
Failed to execute goal org.codehaus.mojo:hibernate3-maven-plugin:2.0:hbm2ddl (generate-ddl) on
project foo: Execution generate-ddl of goal org.codehaus.mojo:hibernate3-maven
plugin:2.0:hbm2ddl failed: Could not determine type for: com.foo.data.FileSystemObject, for
columns: [org.hibernate.mapping.Column(element)] -> [Help 1]
希望有人可以對此有所啟發!
該類型可能有一個Interface,但是它不適用於您配置實體的方式。 為了將兩種Entity類型組合在同一個集合中,它們需要共享一個本身就是Entity的公共超類型。 (也就是說,您需要像FileSystemObject這樣的東西作為它們都繼承的通用超類,該超類在該級別定義ID。)
問題是,請考慮以下查詢:
select c from Folder f, f.contents c where f.name = 'FOLDER' and c.id = 3;
如果同時存在ID為3的文件夾和文件,應該如何知道您想要什么? 這就是為什么如果它們要屬於同一集合,則需要共享一個公共超類。
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