Extending Core Classes

$this->modx->addPackage('extendeduser', $this->extPath .'model/', 'ut_');

<?xml version="1.0" encoding="UTF-8"?>
<model package="extendeduser" baseClass="xPDOObject" platform="mysql" defaultEngine="MyISAM" tablePrefix="ext_">
    <!-- inherit the modx user and extend it -->
    <object class="extUser" table="users" extends="modUser">
        <composite alias="Phones" local="id" foreign="user" cardinality="many" owner="local" />
        <composite alias="Table2" local="id" foreign="user" cardinality="many" owner="local" />
    </object>
 
    <!-- track all user phone numbers -->
    <object table="phone_numbers" extends="xPDOSimpleObject">
        <field key="user" dbtype="int" phptype="integer" null="false" default="0" index="index" />
        <field key="areacode" dbtype="varchar" precision="3" phptype="string" null="false" default="" />
        <field key="number" dbtype="varchar" precision="7" phptype="string" null="false" default="" />
        <aggregate alias="extUser" local="user" foreign="id" cardinality="one" owner="foreign" />
    </object>
 
    <!-- user extension -->
    <object table="table2" extends="xPDOSimpleObject">
        <field key="user" dbtype="int" phptype="integer" null="false" default="0" index="index" />
        <field key="myspaceurl" dbtype="varchar" precision="255" phptype="string" null="false" />        
        <aggregate alias="extUser" local="user" foreign="id" cardinality="one" owner="foreign" />
    </object>
</model>