$m = new \atk4\report\UnionModel($db);
$m->addNestedModel(new Purchase(), [
    'ref_no'        => '[ref_no]',          // mapping fields is optional
    'date'          => '[purchase_date]',   // you can alias like this
    'contact'       => 'concat("From: ", [contractor_from])',
]);                                         // use expressions
$m->addNestedModel(new Sale(), [
    'date'          => '[sale_date]',       // this way we have one column for dates
    'contact'       => 'concat("To: ", [contractor_to])',
]);
$m->setOrder(['date']);                     // sorts resulting union model

$m->addField('date', ['caption' => 'Date', 'type' => 'date']);
     // now add union-based models here.

$m->addField('contact', ['caption' => 'Supplier/Payee']);
$m->addField('ref_no', ['caption' => 'Document No']);
$m->addField('amount', ['caption' => 'Net Amount', 'type' => 'money']);
     // if association in nested model is not explicitly defined, will
     // use field. If no field is found, will use expression: "null"

$m->join('country', 'country_id')
    ->addField('country', 'name');
     // Union Model extends \atk4\data\Model so you can use addField and addExpression


// $m->groupBy('country', ['amount'=>'sum([]')])
// groupBy works like in GroupModel, next example for usage.

$table->setModel($m);

$m = new \smbo\GroupModel(new Sale($db));
$m->groupBy(['contractor_to', 'type'], [      // groups by 2 columns
    'c'                     => 'count(*)',    // defines aggregate formulas for fields
    'qty'                   => 'sum([])',     // [] refers back to qty
    'total'                 => 'sum([amount])', // can specify any field here
]);

$m->addFields(['vat_registered', ['vat_no', 'caption' => 'VAT No']]);
    // add 2 more fields which will bypass aggregation.

$m->getElement('total')->type = 'money';
    // change the type