[hibernate-dev] @OneToOne with @PrimaryKeyJoinColumn(s) vs @MapsId without value element

[Gail Badner]

FYI, I am taking PTO Tuesday, 9/4. I hope to be able to move forward on
this when I return on 9/5.

I see some differences. Some may be expected, but I think there are some
bugs.

For example, suppose we have the following entities:

@Entity
public class Parent {
@Id
private Long id;
}

@Entity
public class ChildPKJC {
@Id
private Long id;

@OneToOne  // note that cascade-persist is not enabled
@PrimaryKeyJoinColumn
private Parent parent;
}

public class ChildMapsId {
@Id
private Long id;

@OneToOne  // note that cascade-persist is not enabled
@MapsId
private Parent parent;
}

-------------------------------------------------------------------------------------------------------------------------------------------

When persisting ChildPKJC:

1) the application must initialize ChildPKJC#id before persisting the
entity [1]; otherwise,  the following exception is thrown:
javax.persistence.PersistenceException:
org.hibernate.id.IdentifierGenerationException: ids for this class must be
manually assigned before calling save():

2) if ChildPKJC#parent is new with an assigned ID, and ChildPKJC#id is
assigned parent's ID, the ChildPKJC Entity is persisted with the parent's
ID, but parent is not persisted.

When persisting ChildMapsId:

1) Hibernate automatically initializes ChildMapsId#id to parent.id [2]

2) if ChildMapsId#parent is new, parent is automatically cascade-persisted
(even though CascadeStyle.PERSIST is not mapped), then the ChildMapsId entity
is persisted.

Are these expected difference? (My guess is yes)

-------------------------------------------------------------------------------------------------------------------------------------------

Foreign key generation:

If ChildPKJC#parent is optional there is no foreign key generated from
ChildPKJC
referencing Parent. [3] If ChildPKJC#parent is not optional, a foreign key
is generated

For ChildMapsId, a foreign key is generated from ChildPKJC referencing
Parent, even if ChildMapsId#parent is optional.

Is this a bug? My guess is that it is.

Adding the following mapping to ChildMapsId#parent works to disable foreign
key generation:
@JoinColumn(foreignKey = @ForeignKey(value = ConstraintMode.NO_CONSTRAINT))
(can be used as a workaround)

-------------------------------------------------------------------------------------------------------------------------------------------

Loading an existing ChildPKJC/ChildMapsId with an optional Parent
association by ID, when there is no Parent entity with the same ID (IIUC,
this is the only way that ChildPKJC#parent or ChildMapsId#parent can be
optional [3]):

For ChildPKJC, the loaded ChildPKJC entity will have a null parent. There
is no need to add @NotFound(IGNORE) to ChildPKJC#parent.

If ChildPKJC#parent is optional, it is always eagerly loaded.

This makes sense, since we cannot create a proxy if there is the
possibility of a null Parent entity.

For ChildMapsId, the loaded value will be null
because ObjectNotFoundException will be thrown when Hibernate tries to load
the Parent entity. Adding @NotFound(IGNORE) to ChildMapsId#parent will
result in ChildMapsId entity being loaded with a null parent association.

Is this expected? If so, then ChildMapsId#parent cannot be optional by
default (without @NotFound(IGNORE).

I think it would make more sense if the ChildMapsId entity is loaded with a
null parent association, consistent with what happens for ChildPKJC. If we
go that route, then ChildMapsId#parent will always have to be loaded
eagerly.

-------------------------------------------------------------------------------------------------------------------------------------------

Please let me know your thoughts on this.

[1] this requirement is documented in Example 178. Derived identifier
@PrimaryKeyJoinColumn with a note that says: "Unlike @MapsId, the
application developer is responsible for ensuring that the identifier and
the many-to-one (or one-to-one) association are in sync as you can see in
the PersonDetails#setPerson method."

[2] Section 2.4.1 Primary Keys Corresponding to Derived Identities of the
spec has this footnote:
[12] If the application does not set the primary key attribute
corresponding to the relationship, the value of that attribute may not be
available until after the entity has been flushed to the database.

[3] Section 11.1.44 PrimaryKeyJoinColumn Annotation has a footnote:
[121]It is not expected that a database foreign key be defined for the
OneToOne mapping, as the OneToOne relationship may be defined as
“optional=true”.


On Fri, Aug 31, 2018 at 1:29 PM, Gail Badner <gbadner at redhat.com> wrote:

> The fix for HHH-12436 involves correcting the foreign key direction for
> "real" one-to-one associations. I've been looking into the ramifications of
> this change because I'm concerned that applications can rely on the old
> (incorrect) foreign key direction.
>
> In the process I've found that Hibernate treats:
>
> @OneToOne
> @PrimaryKeyJoinColumn
> private Employee employee;
>
> differently from:
>
> @OneToOne
> @MapsId
> private Employee employee;
>
> I believe they should be treated consistently. You can see my reasoning
> below. [1]
>
> Before going into details about how they are treated differently, I'd like
> to  get confirmation, in case I am missing some subtlety.
>
> Could someone please confirm this?
>
> Regards,
> Gail
>
> ------------------------------------------------------------
> ---------------------------------
> [1]
>
> In 2.4.1.3 Examples of Derived Identities, Example 4(b) uses MapsId
> without the value element as follows:
>
> @MapsId
> @JoinColumn(name="FK")
> @OneToOne Person patient;
>
> This example has the following footnote:
> "[15] Note that the use of PrimaryKeyJoinColumn instead of MapsId would
> result in the same mapping in this example. Use of MapsId
> is preferred for the mapping of derived identities."
>
> The description has a footnote that says that using PrimaryKeyJoinColumn
> instead of MapsId would result in the same mapping.
>
> In 11.1.45 PrimaryKeyJoinColumns Annotation, Example 2 uses
> @PrimaryKeyJoinColumns as follows:
>
> @OneToOne
> @PrimaryKeyJoinColumns({
>         @PrimaryKeyJoinColumn(name="ID",
>                               referencedColumnName="EMP_ID"),
>         @PrimaryKeyJoinColumn(name="NAME",
>                               referencedColumnName="EMP_NAME")})
> EmployeeInfo info;
>
> This example has the following footnote:
> "[123]Note that the derived identity mechanisms decribed in section
> 2.4.1.1 is now preferred to the use of PrimaryKeyJoinColumn for
> this case."
>
>