Mapping to a Lookup Table

Question

O.K. I have a basic Java question and how to do this the following using
Java/JDO:
I have a database table for a lookup class. It will have the following
structure:
Create Table ProductStatus
Id bigint not null
name char (20);
In C++ I would have an Enumeration Type which constrains the value of the
variable to the set of values in the database.
In Java I was thinking that I would have a ProductStatus class and then in
the Product Class I would have an instance as follows:
public class Product {
ProductStatus status;
But I have now believe that this is wrong...
From offline work I have been told that the correct JDO for what I want
to do is as follows:
<class name="Product">
<extension vendor-name="kodo" key="table" value="JDO_PRODUCT"/>
<extension vendor-name="kodo" key="pk-column" value="ID"/>
<extension vendor-name="kodo" key="class-column" value="JDOCLASS"/>
<extension vendor-name="kodo" key="lock-column" value="none"/>
<field name="currency">
<extension vendor-name="kodo" key="data-column" value="CURRENCY"/>
</field>
<field name="productStatus">
<extension vendor-name="kodo" key="data-column" value="STATUS_ID"/>
</field>
</class>
<class name="ProductStatus">
<extension vendor-name="kodo" key="table" value="JDO_STATUS"/>
<extension vendor-name="kodo" key="pk-column" value="ID"/>
<field name="name">
<extension vendor-name="kodo" key="data-column" value="NAME"/>
</field>
</class>
My two questions are:
1) What do I put for the Java code, both in the product class and for
ProductStatus.... This is more a Java question than JDO... I have been
told I am thinking in too much of a C++ way.... I keep wanting to have
a ProductStatus as a Type Class and then have an instance variable in
the Product class such as status that is of type: ProductStatus.
But I have been told this is not the correct Java way to do things.
How should this be done?
2) I would have expected that the JDO for the ProductStatus field in the
Product Class would have been a FK extension, but I had Abe White help me
with this and he said generated the JDO file I have included.... so how is
the Product.ProductStatus linking to the ProductStatus class ???? With the
current JDO I don't see any link, either by name or by shared ID value....
Obviously I am a total newbie to this and these are basic questions.
Thanks to anyone who has the time to reply.
Brian Smith

Answer 1

Hi Brian --
Java code:
public class Product
     private ProductStatus status;
public class ProductStatus
     private String name;
Metadata:
Exactly like you posted it.
I'm not sure where you're getting confused on how the relation works. It's
straightforward. The STATUS_ID column in the JDO_PRODUCT table holds the
primary key value of the related ProductStatus instance. That's all there is
to it. So, for example, the following Java code:
ProductStatus stat = new ProductStatus ();
stat.setName ("foo");
Product prod = new Product ();
prod.setProductStatus (stat);
pm.currentTransaction ().begin ();
pm.makePersistent (prod);
pm.currentTransaction ().commit ();
Would product SQL like:
INSERT INTO JDO_PRODUCT (ID, STATUS_ID) VALUES (100, 101);
INSERT INTO JDO_STATUS (ID, NAME) VALUES (101, 'foo');
Note that the '101' value for STATUS_ID in the inserted JDO_PRODUCT row
matches the '101' value for the ID (primary key) in the inserted JDO_STATUS
row.
If you wanted to simulate a C++ enumeration and at the same time get rid of
the necessity of a second JDO_STATUS table that seems a little redundant
(why have a table to hold a single piece of data?), you could do it in 2 ways.
The simple way would be to just use static constants in the Product class for
different possible statuses:
public class Product
     public static final String STATUS_XXX = "xxx";
     public static final String STATUS_YYY = "yyy";
     private String status;
And then just have the JDO_PRODUCT table have a VARCHAR column to hold the
status string rather than having a STATUS_ID column that forms a relation to
a separate table.
The second way, which more closely mirrors C++ enumerations, is a little more
complex. You keep the ProductStatus class, but make it non-persistent (note:
the static-instance, private-constructor pattern below is the standard Java
way of doing enums):
public class ProductStatus
     // enumerate all possible product status values
     public static final ProductStatus XXX = new ProductStatus ("xxx");
     public static final ProductStatus YYY = new ProductStatus ("yyy");
     private String name;
     * Convenience method to return a status for a given name.
     public static ProductStatus forName (String name)
          if (XXX.getName ().equals (name))
               return XXX;
          if (YYY.getName ().equals (name))
               return YYY;
          throw new IllegalArgumentException (name);
     * Make constructor private to prevent creation of any status instances
     * with illegal values.
     private ProductStatus (String name)
          this.name = name;
     public String getName ()
          return name;
Then in your Product class, keep a non-persistent ProductStatus instance and
a persistent field with the status name. Use the javax.jdo.InstanceCallbacks
interface to transfer the stored status name to/from the ProductStatus:
public class Product
     implements InstanceCallbacks
     // used internally only; map this to the column in the JDO_PRODUCT table
     // holding the status name for the product; at runtime we'll use JDO
     // lifecycle callbacks to transfer the value to/from a ProductStatus
     // instance
     private String statusName;
     // mark this as non-persistent in the metadata
     private ProductStatus status;
     public void jdoPostLoad ()
          status = ProductStatus.forName (statusName);
     public void jdoPreStore ()
          statusName = (status == null) ? null : status.getName ();
     // rest of InstanceCallbacks methods can be implemented as no-ops...
With Kodo, you could even implement your own FieldMapping to map status strings
to ProductStatus instances, but that's getting really complicated for very
little real advantage.
Hope that helps.