具有@ManyToone关系的QueryDSL投影

Question

@Entity
public class Folder {
    private Long id;
    private String name;
    private String path;
    @ManyToOne
    @JoinColumn(name = "FK_FILE_ID")
    private File file;
}

@Entity
public class File {
    private Long id;
    private String fileName;
}

List<Folder> listFolders = query.from(folder)
.list(Projections.bean(Folder.class, folder.name, folder.file.fileName));

query.list(ConstructorExpression.create(Folder.class, folder.name,
folder.file.fileName));

但是如果可能的话，我想避免这种情况，因为它迫使我定义n个构造函数，为我在投影中想要的n个字段组合定义n个构造函数。

Answer 1

在这种情况下，可以使用嵌套投影

List<Folder> listFolders = query.from(folder)
    .list(Projections.bean(Folder.class, folder.name, 
          Projections.bean(File.class, folder.file.fileName).as("file")));

这里有一个更明确的替代构造函数和bean投影的方法，也适用于这种情况

MappingProjection<Folder> mapping = new MappingProjection<Folder>(Folder.class, folder.name, folder.file.fileName) {
 @Override
 protected Folder map(Tuple row) {
     Folder f = new Folder();         
     f.setName(row.get(folder.name));
     File file = new File();
     file.setFileName(row.get(folder.file.fileName));
     f.setFile(file);
     return f;
 }            

}；