Section : Documentation > Extensions

Extensions : RDBMS SQL Table Naming Support

Plugin

DataNucleus is developed as a plugin-driven framework and one of the components that is pluggable is the support for controlling the naming of table aliases in (some) SQL statements. DataNucleus provides a few options out of the box. The default is alpha-scheme which names tables based on the table-group they are in and the number in that group, so giving names like A0, A1, A2, B0, B1, C0, D0. It also provides a simpler option t-scheme that names all tables as T{number} so T0, T1, T2, T3, etc.

The following sections describe how to create your own SQL Table Namer plugin for DataNucleus.

Interface

Any SQL Table Namer plugin will need to implement org.datanucleus.store.rdbms.sql.SQLTableNamer Javadoc. So you need to implement the following interface

package org.datanucleus.store.rdbms.sql;

import org.datanucleus.store.rdbms.sql.SQLStatement;
import org.datanucleus.store.mapped.DatastoreContainerObject;

public interface SQLTableNamer
{
    /**
     * Method to return the alias to use for the specified table.
     * @param stmt The statement where we will use the table
     * @param table The table
     * @return The alias to use
     */
    public String getAliasForTable(SQLStatement stmt, DatastoreContainerObject table);
}

Implementation

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

  • The SQLStatement that is being constructed and that we create the names for
  • The table that we want to generate the alias for

Lets just go through our default namer scheme to understand how it works

package mydomain;

import org.datanucleus.store.rdbms.sql.SQLTableNamer;
import org.datanucleus.store.rdbms.sql.SQLStatement;
import org.datanucleus.store.mapped.DatastoreContainerObject;

public class MySQLTableNamer implements SQLTableNamer
{
    public String getAliasForTable(SQLStatement stmt, DatastoreContainerObject table)
    {
        if (stmt.getPrimaryTable() == null)
        {
            return "T0";
        }
        else
        {
            return "T" + (stmt.getNumberOfTables() < 0 ? "1" : (stmt.getNumberOfTables()+1));
        }
    }
}

So we simply name the primary table of the statement as T0, and then all subsequent tables based on the number of the table. That was hard!

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_tablenamer">
        <sql-tablenamer name="my-t-scheme" class="mydomain.MySQLTableNamer"/>
    </extension>
</plugin>

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

So now if we define a query using the extension datanucleus.sqlTableNamingStrategy set to my-t-scheme then it will use our table namer.