Hibernate vs DB Table Constraints

Good time.

Say, there are two DB tables: FOLDERS and FOLDER_TYPES.

CREATE TABLE FOLDERS (  
  ID               VARCHAR2(50 BYTE)   NOT NULL, 
  NAME             VARCHAR2(4000 BYTE) NOT NULL, 
  DESCRIPTION      VARCHAR2(4000 BYTE), 
  FOLDER_TYPE_ID   VARCHAR2(50 BYTE) NOT NULL, 
  PARENT_FOLDER_ID VARCHAR2(50 BYTE) 
)

CREATE TABLE FOLDER_TYPES ( 
  ID          VARCHAR2(50 BYTE) NOT NULL, 
  NAME        VARCHAR2(50 BYTE) NOT NULL, 
  DESCRIPTION VARCHAR2(4000 CHAR)
)

As you can see there is a NOT NULL contraint in the FOLDERS table on the FOLDER_TYPE_ID column.

The Hibernate's entities for these tables are the following:

@Entity
@Table(name = "folders")
public class Folder implements Serializable {
   private static final long serialVersionUID = 1856858081142422000L;

   @Id
   @GeneratedValue(generator = "uuid")
   @GenericGenerator(name = "uuid", strategy = "uuid2")
   @Column(name = "[ID]")
   private String id;

   @Column(name = "[NAME]")
   private String name;

   @Column(name = "[DESCRIPTION]")
   private String description;

   @ManyToOne
   @JoinColumn(name = "folder_type_id")
   private FolderType type;

   ...

and

@Entity
@Table(name = "folder_types")
public class FolderType implements Serializable{
   private static final long serialVersionUID = -7992732910184226857L;

    @Id
    @GeneratedValue(generator = "uuid")
    @GenericGenerator(name = "uuid", strategy = "uuid2")
    @Column(name = "[ID]")
    private String id;

    @Column(name = "[NAME]")
    private String name;

    @Column(name = "[DESCRIPTION]")
    private String description;

The problem is: when I remove a Folder object, before the actual deletion from the DB, Hibernate tries to set a null value to the reference (folder type), but there is the constraint, so an exception is thrown. As a workaround I can remove the constraint, but what if it is required to have it, and I can't alter the table (FOLDERS)?

Is there a way to say Hibernate not to update the entity to null references? I've tried setting updatable = false to the @JoinColumn (FolderType type) in the Folder entity, but there has been no luck...