Spring Data JPA cannot save entity into MySQL doc store- error: ids for this class must be manually assigned before calling save()

[Shahza Syed]

Hello,

My document store looks like this:

CREATE TABLE ATTACHMENT (
ATTACHMENT_ID varchar(32) GENERATED ALWAYS AS (json_unquote(json_extract(DOC,'$.attachment_id'))) STORED NOT NULL,
DOC json NOT NULL,
CONSTRAINT PK_ATTACHMENT_ATTACHMENT_ID PRIMARY KEY (ATTACHMENT_ID)
);

Note that my Attachment domain object (which will be the doc inserted) has an attachment_id field in it.
This is the entity I'm trying to insert (without the setters and getters):

@Entity(name = "Attachment")
@Table(name = "ATTACHMENT")
public class AttachmentEntity {

@Id
@Column(name = "ATTACHMENT_ID")
private String id;

@Convert(converter = JpaAttachmentConverterJson.class)
@Column(name = "doc", nullable = false)
private Attachment doc;

...
}

Here is the interface that implements JpaRepository:

public interface JpaAttachmentEntityRepository extends JpaRepository<AttachmentEntity, String> {
}

In another class I simply call the above repository's .save() method, passing in an AttachmentEntity, like so:
this.jpaAttachmentEntityRepository.save(attachmentEntity);

Here is the important part of the error message:

org.hibernate.id.IdentifierGenerationException: ids for this class must be manually assigned before calling save(): uk.co.myapp.repository.jpa.AttachmentEntity

If I try to set the id field before calling .save then this error is returned: java.sql.SQLException: The value specified for generated column 'ATTACHMENT_ID' in table 'attachment' is not allowed.

Note: Since the ATTACHMENT_ID field is generated when a record is entered into the doc store and not auto incremented, I got rid of a @GeneratedValue annotation under @Id in the entity (keeping this did not work either).

How can I successfully insert such an entity?

Thank you in advance.