Problem with Oracle 7.3 and WO3.5.1

[METRiX Support <support@metrixtech.com>]

I have a problem with a WO3.5.1 application running against an Oracle 7.3
database.

I have 3 important tables in Oracle being administrated by my WebObjects
application. (Table A) and (Table B) contain data and (Table C) is a join
table allowing Many-To-Many relationships between (Table A) and (Table B).

I can insert and delete records from (Table A).
I can insert and delete records from (Table B).
I can delete records from (Table C).
I cannot, however, insert "join" records into (TABLE C).

Instead of inserting a record into the join table (Table C) I receive this
error:
_primaryKeyForObject:: EODatabaseContext 0x5be890 is unable to obtain a
primary
                   key for InvestigatorSite object 0x6169b0


I am wondering:
 what to do to fix the problem, 
interpretations of what might be causing this problem,
and strategies for avoiding it in the future.

Thanks in advance for your help,
DB




--------------------------------
METRiX Technologies Support Desk
WebObjects Applications
301-977-9553 Ext. 15