Interface

Any SQL operation plugin will need to implement org.datanucleus.store.rdbms.sql.operation.SQLOperation . So you need to implement the following interface

import org.datanucleus.store.rdbms.sql.method;

public interface SQLOperation
{
    /**
     * Return the expression for this SQL function.
     * @param expr Left hand expression
     * @param expr2 Right hand expression
     * @return The SQL expression for the operation
     */
    public SQLExpression getExpression(SQLExpression expr, SQLExpression expr2);
}

Implementation

So there is only one method to provide in your implementation. The arguments to this are

• The expression on the left hand side of the operation. So if you have {expr1} {operation} {expr2} then the first argument will be {expr1}
• The expression on the right hand side of the operation. So if you have {expr1} {operation} {expr2} then the first argument will be {expr2}

So if we wanted to support modulus (%) (so something like expr1 % expr2) and wanted to use the SQL function MOD to provide this then we define our class as

package mydomain;

import java.util.ArrayList;

import org.datanucleus.exceptions.NucleusException;
import org.datanucleus.store.rdbms.sql.operation.SQLOperation;
import org.datanucleus.store.rdbms.sql.expression.SQLExpression;
import org.datanucleus.store.rdbms.sql.expression.NumericExpression;

public class MyModOperation implements SQLOperation
{
    public SQLExpression getExpression(SQLExpression expr, SQLExpression expr2)
    {
        ArrayList args = new ArrayList();
        args.add(expr);
        args.add(expr2);
        return new NumericExpression("MOD", args);
    }
}

So in this implementation when the user includes {expr1} % {expr2} this is translated into the SQL MOD({expr1}, {expr2}) which will certainly work on some RDBMS. Obviously you could use this extension mechanism to support a different underlying SQL function.

Plugin Specification

So we now have our custom SQL method and we just need to make this into a DataNucleus plugin. To do this you simply add a file plugin.xml to your JAR at the root. The file plugin.xml should look like this

<?xml version="1.0"?>
<plugin id="mydomain" name="DataNucleus plug-ins" provider-name="My Company">
    <extension point="org.datanucleus.store.rdbms.sql_operation">
        <sql-operation name="mod" datastore="hsql"
            evaluator="mydomain.MyModOperation"/>
    </extension>
</plugin>

Note that you also require a MANIFEST.MF file as per the Extensions Guide.

So we defined calls to an operation mod for the datastore hsql to use our evaluator. Simple! Whenever this operation is encountered in a query from then on for the HSQL database it will use our operation evaluator.