[ExtJS] - How to fill in a grid with datas from 2 different tables ?

<?xml version="1.0" encoding="UTF-8"?>
<model package="cispeo" baseClass="xPDOObject" platform="mysql" defaultEngine="MyISAM" phpdoc-package="cispeo">
		
    <object class="cpoCompany" table="cispeo_companies" extends="xPDOSimpleObject">
        
		<field key="name" dbtype="varchar" precision="80" phptype="string" null="false" default="" index="index" />
		<field key="office" dbtype="varchar" precision="80" phptype="string" null="true" default="" index="index" />
		<field key="description" dbtype="text" phptype="text" null="true" default="" />
	
		<composite alias="Projects" class="cpoProject" local="id" foreign="owner" cardinality="many" owner="local" />
    </object>
    <object class="cpoProject" table="cispeo_projects" extends="XPDOSimpleObject">
		<field key="owner" dbtype="int" precision="10" phptype="integer" attributes="unsigned" null="false" default="0" index="index" />
		<field key="name" dbtype="varchar" precision="50" phptype="string" null="false" default="" index="index" />
		<field key="description" dbtype="text" phptype="text" null="true" default="" />
		<field key="status" dbtype="tinyint" precision="1" phptype="integer" attributes="unsigned" null="false" default="0" index="index" />
		
		<aggregate alias="Company" class="cpoCompany" local="owner" foreign="id" cardinality="one" owner="foreign" />
		
	</object>
	
	<!-- An Event -->
</model>

$tariffs = $modx->getCollection('Tariff', $c);
/* iterate */
$list = array();
foreach ($tariffs as $tariff) {
    $property = $tariff->getOne("property");
    $tariffArray = $tariff->toArray();
    $tariffArray["property"] = $property->get("name");
    $list[]= $tariffArray;
}