gxText

<model package="gxTest" baseClass="xPDOObject" platform="mysql" defaultEngine="MyISAM" version="1.1">
    
    <object class="gxBox" table="gx_box" extends="xPDOSimpleObject">
        <field key="title" dbtype="varchar" precision="255" phptype="string" null="false"/>
        <field key="properties" dbtype="text" phptype="string" null="false"/>
        
        <field key="class_alias" dbtype="varchar" precision="255" phptype="string" null="false"/>
        
		<composite alias="gxBoxWater"       class="gxBoxWater"      local="id" foreign="id" cardinality="one" owner="local"/>
		<composite alias="gxBoxAutomobil"   class="gxBoxAutomobil"  local="id" foreign="id" cardinality="one" owner="local"/>
    </object>
    
    <object class="gxBoxWater" table="gx_box_water" extends="xPDOSimpleObject">
        <field key="pressure" dbtype="int" precision="11" phptype="integer" null="false"/>
        <field key="speed" dbtype="int" precision="11" phptype="integer" null="false"/>
        <field key="beasts" dbtype="varchar" precision="255" phptype="string" null="false"/>
        
        <aggregate alias="gxBox" class="gxBox" local="id" foreign="id" cardinality="one" owner="foreign"/>
    </object>
    
    <object class="gxBoxAutomobil" table="gx_box_sky" extends="xPDOSimpleObject">
        <field key="wheels" dbtype="int" precision="11" phptype="integer" null="false"/>
        <field key="has_truck" dbtype="tinyint" precision="1" phptype="boolean" null="false"/>
        <field key="truck_volume" dbtype="int" precision="11" phptype="integer" null="false"/>
        
        <aggregate alias="gxBox" class="gxBox" local="id" foreign="id" cardinality="one" owner="foreign"/>
    </object>
    
</model>

usage

<?php

$waterbox = $modx->newObject('gxBox',array(
    'title' => 'FirstBoxOfWater',
    'class_alias' => 'gxBoxWater'
));
$waterbox->addOne($modx->newObject('gxBoxWater',array(
    'pressure' => '1.34',
)),'gxBoxWater');

$automobil = $modx->newObject('gxBox',array(
    'title' => 'SecondBoxOfAutomobil',
    'class_alias' => 'gxBoxAutomobil'
));

$automobil->addOne($modx->newObject('gxBoxAutomobil',array(
    'wheels' => 5/*=D*/
)),'gxBoxWater');

$waterbox->save();
$automobil->save();