CODEWITHSUNDEEP
javahibernatejpa
Al Koch
Al Koch

Reputation: 423

How configure for a (Optional) OneToOne Composite Primary Key?

I am using Hibernate and have two tables, STUDENTS and DORM_ROOMS, that are related with a composite key:

STUDENTS table:
CAMPUS(String)      Part of Composite Key
STUDENT_ID (int)    Part of Composite Key
NAME (String)
...

DORM_ROOMS table:
CAMPUS(String)      Part of Composite Key
STUDENT_ID (int)    Part of Composite Key
ROOM_NUMBER(int)
...

The relationship is one to one because a student can be associated with exactly one dorm room and and a dorm room is associated with one student (wow - a private room!). However, not all students have a dorm room.

My initial code (stripped down) looks like:

FOR STUDENTS:
@Embeddable
public class StudentsPK implements Serializable {
@Column(name = "CAMPUS")
private String Campus;
@Column(name = "STUDENT_ID")
private String StudentID;
...
}


@Entity
@Table(name = "STUDENTS")
public class Students implements Serializable {
@EmbeddedId
private StudentsPK studentsPK;
...
}


FOR DORM_ROOMS:
@Embeddable
public class DormRoomsPK implements Serializable {
@Column(name = "CAMPUS")
private String Campus;
@Column(name = "STUDENT_ID")
private String StudentID;
...
}


@Entity
@Table(name = "DORM_ROOMS")
public class DormRooms implements Serializable {
@EmbeddedId
private DormRoomsPK dormRoomsPK;
...
}

Assume that the database schema is already defined and created. In particular, CAMPUS+STUDENT_ID is a PK for STUDENTS and CAMPUS+STUDENT_ID is a FK for DORM_ROOMS that serves as the PK in that table. At this point I can successfully insert a row into STUDENTS and a row into DORM_ROOMS. I can also retrieve any student from STUDENTS even if the student does not have a dorm room. However, I have not yet "informed" Hibernate about the relationship between the two tables. That is where I am confused.

I tried to "relate" the two tables by using a "JOIN" annotation but I discovered that this causes any attempt to fetch a student that has no dorm room to return an empty result set. I suppose that makes since if "JOIN" states that the tables are to always be viewed as joined then joining a student having no dorm room with no matching rows in the DORM_ROOMS table would result in an empty result set.

Since using a "JOIN" annotation doesn't work, how do I modify my code to describe the relationship between the two tables but still allow me to fetch students that have no matching dorm rooms?

Thank you.

Upvotes: 0

Views: 177

Answers (2)

Al Koch
Al Koch

Reputation: 423

Found the problem! I discovered that in a @OneToOne relationship with a composite key, using a separate FK class to manage the composite key in both entities causes the error. The problem is shown in my original posting where I define and use StudentsPK and DormRoomsPK! Once I changed to use a single "PK" class instead of these two my problem was eliminated. (This doesn't appear to be a well documented requirement!)

Upvotes: 1

mkobit
mkobit

Reputation: 47249

It sounds like you are looking for the @OneToOne annotation, which also has the ability to specify if the relationship is optional. There are some examples described in the JBoss JPA 2.1 docs, here is one of them.

Example 3: One-to-one association from an embeddable class to another entity.

@Entity
public class Employee {
  @Id int id;
  @Embedded LocationDetails location;
  ...
}

@Embeddable
public class LocationDetails {
  int officeNumber;
  @OneToOne ParkingSpot parkingSpot;
  ...
}

@Entity
public class ParkingSpot {
  @Id int id;
  String garage;
  @OneToOne(mappedBy="location.parkingSpot") Employee assignedTo;
}

Upvotes: 1

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