This involves one-to-one and many-to-one associations that are non-optional, where the associated entity ID is non-null, but there is no entity with that ID. This can only happen when there is no foreign key constraint on the association ID.

There are 3 cases where Hibernate will load an entity with a null association, even though the the association is non-optional:

1) The mapping includes @NotFound(action = NotFoundAction.IGNORE). For example:

@Entity
public void Employment {
    @Id
    private long id;

    @ManyToOne(optional=false)
    @NotFound(action = NotFoundAction.IGNORE)
    @JoinColumn(foreignKey = @ForeignKey(ConstraintMode.NO_CONSTRAINT))
    private Employee employee;

@NotFound(action = NotFoundAction.IGNORE) is not compatible with a non-optional association. Hibernate should either throw an exception when this is detected. At the very least, @NotFound(action = NotFoundAction.IGNORE) should be ignored, and a warning should be logged.

2) The mapping has @OneToOne(optional = false) @MapsId @Fetch(value = FetchMode.SELECT)}, with {{@JoinColumn used to override the column name. For example:

@Entity
public void Employment {
    @Id
    private long id;

    @OneToOne(optional=false)
    @MapsId
    @JoinColumn(name = "FK", @JoinColumn(foreignKey=@ForeignKey(NO_CONSTRAINT)))
    @Fetch(value = FetchMode.SELECT)
    private Employee employee;
}

3) The mapping has @OneToOne(optional = false) with @PrimaryKeyJoinColumn, and @Fetch(value = FetchMode.SELECT). For example:

@Entity
public void Employment {
    @Id
    private long id;

    @OneToOne(optional = false)
    @PrimaryKeyJoinColumn(foreignKey = @ForeignKey(ConstraintMode.NO_CONSTRAINT))
    @Fetch(value = FetchMode.SELECT)
    @NotFound(action = NotFoundAction.EXCEPTION)
    private Employee employee;
}

NOTE: Currently, the foreign key constraint cannot be disabled via @JoinColumn(foreignKey=@ForeignKey(NO_CONSTRAINT)) for 2) or 3) due to HHH-12975 Open . When testing, the foreign key will need to be dropped, or the Employment table will have to be dropped and recreated without the foreign key.

For cases 2) and 3), Hibernate should return null for loaded Employment, since it is impossible to have a non-null Employment.employee.