Building relationships that have no foreign key

Question

Hi,
I have a situation where the database team have provided me a schema
something like that shown below:
PERSON
|========================================|
| PK | NAME | SSNUM | MGR |
|========================================|
| 1 | john | 123-45-6789 | 3 |
| 2 | fred | 987-65-4321 | 3 |
| 3 | alice | 111-22-3333 | NULL |
| ... |
|========================================|
COMPANY
|=================|
| PK | NAME |
|=================|
| 1 | acme |
| 2 | bcme |
| 3 | ccme |
| |
|=================|
ATTRIBUTE
|========================================|
| PK | SOURCE_ID | SOURCE_TYPE | VALUE |
|========================================|
| 1 | 1 | P | one |
| 2 | 1 | C | two |
| ... |
|========================================|
PERSON and COMPANY are application objects. A PERSON or a COMPANY can
have an ATTRIBUTE tagged to it. Now, there is not a foreign key
relationship from PERSON/COMPANY to ATTRIBUTE. The relationship can be
derived by understanding that ATTRIBUTE.SOURCE_ID is the PK value for
either PERSON or COMPANY, and ATTRIBUTE.SOURCE_TYPE states whether its a
PERSON (P) or COMPANY (C).
The database guys say this is a common approach to modelling data that
is not a "first order" property of a class, which I guess may be up for
debate. In any case....
I can't model this using objects and relationships using KODO. So, my
question is, how would you approach this?
Thanks,
Mike.
PS As an aside, if there was a way in the JDO metadata to "hardcode" a
value for one or more of the columns in a foreign key relationship, it
would help a lot. Then I could build a navigable link from PERSON to
ATTRIBUTE, where the SOURCE_ID data column is PERSON.PK and the
SOURCE_TYPE is the string literal 'P'.

Answer 1

That certainly is a unique problem... and certainly not a sort of model I
would encourage...
however, you should be able to accomplish this in Kodo with a non-direct
relationship... i.e. Attributes has a reference-id not a reference. So
Attribute may say look like this:
private int referenceId = -1;
private char type;
public void getSource ()
Object oid = null;
if (referenceId < 0)
     return null;
switch (type)
case 'P':
     oid = new PersonId (referenceId);
     break;
case 'C':
     oid = new CompanyId (referenceId);
     break:
default: return null;
return JDOHelper.getPeristenceManager (this).getObjectById (oid, true);
public void setSource (Person p)
PersonId pid = (PersonId) JDOHelper.getObjectId (p);
refernceId = pid.id;
type = 'P';
Now this would get a little more complicated for objects which
haven't been made persistent, but can simply be accomplished by using
javax.jdo.InstanceCallbacks and preStore () and using a transient field
Object source. Using non-application identity, would require casting oids to
our ObjectIds.Id (in our JavaDoc). Actually, as I think about it, it may
be better to do this all in a transient state:
public Attribute
     private transient Object src;
     private int referenceId;
     private char type;
     public void setSource (Person p)
          src = p;
     public Object getSource ()
          if (src != null || JDOHelper.getPersistenceManager (this) == null)
               return src;
          /// same as previous example
          return JDOHelper.getPersistenceManager (this).getObjectById (oid);
     public void preStore () // instancecallbacks
          PersonId id = (PersonId) JDOHelkper.getObjectId (src);
          referenceId = id.id;
          type = 'P';
Again this isn't 100% solid code but a suggestion to get you started on
your problem.
While 3.0 will support more complex mappings, it won't get around the
multiple types per field problem... but this could be solved via
interfaces (e.g. public interface AttributeSource), though I'd have to
defer to our 3.0 architect on that one.
On Fri, 14 Feb 2003 16:15:04 +0200, Mike Hogan wrote:
Hi,
I have a situation where the database team have provided me a schema
something like that shown below:
PERSON
|========================================|
| PK | NAME | SSNUM | MGR |
|========================================|
| 1 | john | 123-45-6789 | 3 |
| 2 | fred | 987-65-4321 | 3 |
| 3 | alice | 111-22-3333 | NULL |
| ... |
|========================================|
COMPANY
|=================|
| PK | NAME |
|=================|
| 1 | acme |
| 2 | bcme |
| 3 | ccme |
| |
|=================|
ATTRIBUTE
|========================================|
| PK | SOURCE_ID | SOURCE_TYPE | VALUE |
|========================================|
| 1 | 1 | P | one |
| 2 | 1 | C | two |
| ... |
|========================================|
PERSON and COMPANY are application objects. A PERSON or a COMPANY can
have an ATTRIBUTE tagged to it. Now, there is not a foreign key
relationship from PERSON/COMPANY to ATTRIBUTE. The relationship can be
derived by understanding that ATTRIBUTE.SOURCE_ID is the PK value for
either PERSON or COMPANY, and ATTRIBUTE.SOURCE_TYPE states whether its a
PERSON (P) or COMPANY (C).
The database guys say this is a common approach to modelling data that
is not a "first order" property of a class, which I guess may be up for
debate. In any case....
I can't model this using objects and relationships using KODO. So, my
question is, how would you approach this?
Thanks,
Mike.
PS As an aside, if there was a way in the JDO metadata to "hardcode" a
value for one or more of the columns in a foreign key relationship, it
would help a lot. Then I could build a navigable link from PERSON to
ATTRIBUTE, where the SOURCE_ID data column is PERSON.PK and the
SOURCE_TYPE is the string literal 'P'.--
Stephen Kim
[email protected]
SolarMetric, Inc.
http://www.solarmetric.com