How to map M-N-O relationship with JDO

Question

Hi,
I am relatively new to JDO so any help would be appreciated. I have a
database that has 4 tabels, Item, Section, Mechanism and
MechanismItemSection. MechanismItemSection is a mapping table between the
rest of the three tables. It contains the primary keys of all the three
tables(which makes the primary key for this table) and few more text
fields. In order to map it to the JDO objects, i created 4 classes for
each table and mapped the fields of the classes to the table.
However, i am having problem with the MechanismItemSection table. The
primary key of the table constitutes of three fields. How do i define an
objectID for such a table? Is this the right way of doing it or there is a
better way then this without having to change the schema for the database?
Thanks in advance

Answer 1

Dear Atif,
In order to customize primary keys you will want to use application
identity, whereby your application has complete control over the key
values.
A good overview of JDO identity types can be seen at:
http://www.solarmetric.com/Software/Documentation/2.3.0/jdo_overview_pc_identity.html
For example, in your case, you class might look like this (note that
this code is not tested at all):
public class MechanismItemSection
     public int myPK1;
     public short myPK2;
     public double myPK3;
     public String someStringField1;
     public String someStringField2;
     // ... etc ...
     // application identity class, implemented as an inner
     // class for convenience. Note that field names and types
     // must be public, and must match fields defined in the
     // MechanismItemSection class.
     public static final class Id
          public int myPK1;
          public short myPK2;
          public double myPK3;
          public Id ()
          public Id (String serialized)
               StringTokenizer tok = new StringTokenizer (serialized, ":");
               myPK1 = Integer.parseInt (tok.nextToken ());
               myPK2 = Short.parseShort (tok.nextToken ());
               myPK3 = Double.parseDouble (tok.nextToken ());
          public String toString ()
               String serializeString = myPK1 + ":" + myPK2 + ":" + myPK3;
               return serializeString;
          public int hashCode ()
               return (int)(myPK1 + myPK2 + myPK3);
          public boolean equals (Object other)
               Id otherId = (Id)other;
               return otherId.myPK1 == myPK1
               && otherId.myPK2 == myPK2
                    && otherId.myPK3 == myPK3;
In article <[email protected]>, ...Atif... wrote:
Hi,
I am relatively new to JDO so any help would be appreciated. I have a
database that has 4 tabels, Item, Section, Mechanism and
MechanismItemSection. MechanismItemSection is a mapping table between the
rest of the three tables. It contains the primary keys of all the three
tables(which makes the primary key for this table) and few more text
fields. In order to map it to the JDO objects, i created 4 classes for
each table and mapped the fields of the classes to the table.
However, i am having problem with the MechanismItemSection table. The
primary key of the table constitutes of three fields. How do i define an
objectID for such a table? Is this the right way of doing it or there is a
better way then this without having to change the schema for the database?
Thanks in advance
Marc Prud'hommeaux [email protected]
SolarMetric Inc. http://www.solarmetric.com
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