How to map a collection Map<String,List<String>> type using Hibernate

Question

The question was to persist the following class using Hibernate .

Public class Album{
Private int albumid;
Private String aname;
Private Map<String,List<String>> photos;
}

I have Tried this

@Entity
public class Album {
    @Id
    private int albumId;
    @Column(name= "Album_Name")
    private String aname;
    
    @ElementCollection
    @MapKeyColumn(name= "Event_Name")
    @Column(name="Values")
    private Map<String, List<String>> photos;

But it is showing errors such as


    Exception in thread "main" org.hibernate.MappingException: Could not determine type for: java.util.List, at table: Album_photos, for columns: [org.hibernate.mapping.Column(Values)]
        at org.hibernate.mapping.SimpleValue.getType(SimpleValue.java:336)
        at org.hibernate.mapping.SimpleValue.isValid(SimpleValue.java:310)
        at org.hibernate.mapping.Collection.validate(Collection.java:315)
        at org.hibernate.mapping.IndexedCollection.validate(IndexedCollection.java:89)
        at org.hibernate.cfg.Configuration.validate(Configuration.java:1362)
        at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1849)
        at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1928)
        at com.wipro.Insert.main(Insert.java:17)

There is no such annotations,you have to create new class for photos and use hibernate mappings like @OnetoMany ,this will create 2 tables. — Codemaster
– Codemaster, Commented Apr 10, 2021 at 18:22
Kindly check the link stackoverflow.com/questions/3774198/… — Anmol Bhaskar
– Anmol Bhaskar, Commented Apr 10, 2021 at 18:24

Davide D'Alto · Accepted Answer · 2021-04-10 21:32:44Z

This mapping is not going to work in Hibernate ORM.

The reason is that you are trying to have two nested collections of elements and this is not supported by Hibernate ORM (first collection is the Map and the second collection is the the List).

You have to use entities.

You can obtain something similar to an @ElementCollection with the following mapping:


@Entity
public static class PhotoEvent {
    @Id
    @Column(name = "Event_Name")
    public String eventName;

    @ElementCollection
    @Column(name = "`Values`")
    public List<String> values;

    @ManyToOne
    public Album album;
...
// getter/setter/hascode/equals...
}


@Entity
public static class Album {
    ...
    
    @OneToMany(mappedBy = "album", fetch = FetchType.EAGER, cascade = CascadeType.ALL, orphanRemoval = true)
    @MapKey(name = "eventName")
    public Map<String, PhotoEvent> photoEvents;
    
    ...
}

Note that I set FetchType.EAGER because it emulates an @ElementCollection but you will probably want to set it to LAZY (the default).

You will find more details about this type of mapping in the Hibernate ORM documentation.

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