Site site = new Site();
SiteRuntime rt = new SiteRuntime();
site.setRuntime(rt);
rt.setSite(site);
em.persist(site); //ok

em.createQuery("delete from Site s where s.name='text'").executeUpdate(); // exception, see below.

javax.persistence.EntityExistsException: org.hibernate.exception.ConstraintViolationException: could not execute update query
        at org.hibernate.ejb.AbstractEntityManagerImpl.throwPersistenceException(AbstractEntityManagerImpl.java:612)
        at org.hibernate.ejb.QueryImpl.executeUpdate(QueryImpl.java:58)
...
Caused by: com.mysql.jdbc.exceptions.MySQLIntegrityConstraintViolationException: Cannot delete or update a parent row: a foreign key constraint fails (`db1/site_runtime`, CONSTRAINT `FK2627A2C06DE06746` FOREIGN KEY (`SITE_ID`) REFERENCES `site` (`SITE_ID`))
        at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1011)
...

@Entity
public class Site implements Serializeable {
   
    public Site() {}

    @Id @GeneratedValue @Column(name = "SITE_ID", nullable = false)
    private Long id;
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
   
    @Column(name = "SITE_NAME", nullable = true)
    private String name;
    public String getName() { return this.name; }
    public void setName(String siteName) { this.name = siteName; }

// if I change the relationship below to OneToMany, all is OK,
// but then I'm forced to change the type of the member to some
// collection, and I wish to avoid that...   
    @OnDelete(action=OnDeleteAction.CASCADE)
    @OneToOne(cascade=CascadeType.ALL, fetch=FetchType.LAZY, mappedBy="site")
    private SiteRuntime runtime;
    public SiteRuntime getRuntime() { return runtime; }
    public void setRuntime(SiteRuntime runtime) { this.runtime = runtime; }
...
}


@Entity
public class SiteRuntime implements Serializeable {
   
    public SiteRuntime() {
    }
   
    @Id
    @GeneratedValue
    @Column(name = "SITE_RUNTIME_ID", nullable = false)
    private Long id;
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
   
   
    @OneToOne(fetch = FetchType.LAZY, optional=false)
    @JoinColumn(name="SITE_ID", nullable=false)
    private Site site;
    public Site getSite() { return site; }
    public void setSite(Site site) {
        this.site = site;
        if (site != null) this.siteId = site.getId();
    }

    @Column(name="SITE_ID", nullable=false, insertable=false, updatable=false)
    private Long siteId;
    public Long getSiteId() { return siteId; }
    public void SiteId(Long siteId) { this.siteId = siteId; }

...
}

<property name="hibernate.archive.autodetection" value="hbm"/>

public class Item implements Serializable {
    private Long id;
    public Long getId() { return this.id; }
    public void setId(Long id) { this.id = id; }
   
    private ItemInfo itemInfo;
    public ItemInfo getItemInfo() { return itemInfo; }
    public void setItemInfo(ItemInfo itemInfo) { this.itemInfo = itemInfo; }
}

<class name="pu1.Item" table="ITEM">
    <id name="id" column="ITEM_ID">
        <generator class="native"/>
    </id>
    <one-to-one name="itemInfo" property-ref="item" cascade="all" />
</class>

public class ItemInfo implements Serializable {
    private Long id;
    public Long getId() { return this.id; }
    public void setId(Long id) { this.id = id; }

    private Item item;
    public Item getItem() { return this.item; }
    public void setItem(Item item) { this.item = item; }
}

<class name="pu1.ItemInfo" table="ITEM_INFO">
    <id name="id" column="ITEM_INFO_ID">
        <generator class="native"/>
    </id>
    <many-to-one name="item" column="ITEM_ID" unique="true"/>
</class>

CREATE TABLE  `test`.`item` (
  `ITEM_ID` bigint(20) NOT NULL auto_increment,
  PRIMARY KEY  (`ITEM_ID`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=latin1;

CREATE TABLE  `test`.`item_info` (
  `ITEM_INFO_ID` bigint(20) NOT NULL auto_increment,
  `ITEM_ID` bigint(20) default NULL,
  PRIMARY KEY  (`ITEM_INFO_ID`),
  UNIQUE KEY `ITEM_ID` (`ITEM_ID`),
  KEY `FK2836965A74280EDC` (`ITEM_ID`),
  CONSTRAINT `FK2836965A74280EDC` FOREIGN KEY (`ITEM_ID`) REFERENCES `item` (`ITEM_ID`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=latin1;

em.createQuery("delete from Site s where s.name='text'").executeUpdate();