Hibernate: fetch only a specific field for ManyToOne relation (do not embed full entity)

Question

Is it possible in Hibernate to embed only one property of a foreign object into another entity, not the full object?

This is my Problem:

@Entity
@Table(name = "ELEM")
public class Element {

    @Id
    private long id;

    @Column(name = "CODE")
    private String code;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "FOLDR_ID", nullable = false)
    private Folder folder;
}   

@Table(name = "FOLDR")
public class Folder {

    @Id
    private long id;

    @Column(name = "LABEL")
    private String label;

    // dozens of other columns
    // ...
}

This works fine, but now, in Element I want to have only the label, not the complete Folder object. Something like this:

@Entity
@Table(name = "ELEM")
public class Element {

    @Id
    private long id;

    @Column(name = "CODE")
    private String code;

    // HOW TO ANNOTATE THIS?
    private String folderLabel;
}

Why do I want this? Because I think this should be more performant. There are less objects created and less columns fetched.

I tried to use @SecondaryTable, but learned that this only works for one-to-one associations, i.e., Folder.id corresponds to Element.id, which is not the case here.

Can this be done somehow in Hibernate, or is the full foreign object the only chance?

Answer 1

You could use a Hibernate ResultTransformer (e.g. AliasToBeanResultTransformer) to return a custom projection (POJO) based on your needs.

The following Link should help you understand what I mean.

https://vladmihalcea.com/why-you-should-use-the-hibernate-resulttransformer-to-customize-result-set-mappings/

Answer 2

    @Entity
    @Table(name = "ELEM")
    public class Element {

        @Id
        private long id;

        @Column(name = "CODE")
        private String code;

    //Model to get the @ManyToOne object. Do not use it yet, not directly atleast. Most of the times, it leads to Json Infinite Recursion
    @ManyToOne(bla bla) 
    @JsonIgnore
    private Map<K, V> folder 

    //Getters and Setters for folder. Use the @Transient method below for the required field to show up when you deserialize Json or other access methods. 

    //Solution:
        // HOW TO ANNOTATE THIS? Do not use it this way
        private String folderLabel;

//Get it this way
    @Transient 
    public String getFolderLabel(){
    return getFolder().getFolderName();
    }


    }