Spring Boot 从模型中提取有限的字段

Question

I am using JPA and below is my Dto for AssetItem. Here in assetItemDocsCollection i am trying to fetch Collection. But i dont want all fields from AssetItemDocs. For example i dont want documentByte as this field will be huge. I just want all fields leaving this field, how can i do this in less code.

public class AssetItemDto extends AbstractDto<Long> {

    private Collection<AssetItemDocs> assetItemDocsCollection;
}

public class AssetItemDocs implements Serializable {

    private static final long serialVersionUID = 1L;
    @Id
    @Basic(optional = false)
    @Column(name = "id")
    private Integer id;
    @Column(name = "name")
    private String name;
    @Lob
    @Column(name = "document_byte")
    private byte[] documentByte;
    @Column(name = "creation_date")
    @Temporal(TemporalType.DATE)
    private Date creationDate;
    @JoinColumn(name = "asset_item_id", referencedColumnName = "id")
    @ManyToOne
    private AssetItem assetItemId;
}

Answer 1

This is typically what projections are made for: retrieving only part of the entity's attributes. There are several ways to do it, but for instance:

Instead of having AssetItemDocs entities in the Collection of your DTO, create another DTO with all fields of AssetItemDocs that you want to fetch (that is all except documentByte ). That DTO must have a constructor with all theses fields as parameters (type and name must match those declared in the entity).

Then in your repository, write a query method whose return type is a Collection of that DTO. If your query is derivable by its method name, it will work as is, but if your method is written using a JPQL query in a @Query annotation, you'll need to explicitly call the DTO constructor in the select cause with this kind of syntax: SELECT new full.package.MyDTO(entity.attribute1, entity.attribute2) FROM MyEntity entity .

Then you just have to include the Collection returned by the repository in your AssetItemDto .

NB: although it would be possible to make a projection using the same entity class instead of a new DTO, I personally think it's a bad practice to expose entity objects in a DTO.