function SearchForm() {

$searchText = isset($this->Query) ? $this->Query : 'Znajdź produkt';
$fields = new FieldSet(
new TextField("Search", "", $searchText)
);
$actions = new FieldSet(
new FormAction('results', 'szukaj')
);

$form = new SearchForm($this, "SearchForm", $fields, $actions);
$form->classesToSearch(array('SiteTree', 'Product'));

return $form;
}

   /**
    * Classes to search
    */   
   protected $classesToSearch = array("SiteTree", "Product", "File");

function classesToSearch($classes) {
$illegalClasses = array_diff($classes, array('SiteTree', 'Product', 'File'));
..
$legalClasses = array_intersect($classes, array('SiteTree', 'Product', 'File'));

public function searchEngine(..............) {
$extraFilters = array('SiteTree' => '', 'Product'=>'', 'File' => '');

.....

if($keywords) {
$match['Product'] = "MATCH (Content, TechnicalInformation) AGAINST ('$keywords') AND ClassName = 'Product'";
$relevance['Product'] = "MATCH (Content, TechnicalInformation) AGAINST ('$relevanceKeywords' IN BOOLEAN MODE)";
}

    // Generate initial queries and base table names
    $baseClasses = array('SiteTree' => '', 'Product'=>'', 'File' => '');

foreach($this->classesToSearch as $class) {
...
$baseClasses['Product'] = '`Product_live`';
}

    // Make column selection lists
    $select = array(
'Product' => array("ClassName", "$baseClasses[Product].ID","_utf8'' AS ParentID","Content","TechnicalInformation", "$relevance[Product] AS Relevance", "NULL AS CanViewType"),
)

foreach($queries as $query) {
      $querySQLs[] = $query->sql();
      //$totalCount += $query->unlimitedRowCount(); (comented)
}

}
}

[User Error] Couldn't run query: SELECT ClassName, `Product_live`.ID, _utf8'' AS ParentID, Content,TechnicalInformation, MATCH (Content,TechnicalInformation) AGAINST ('something' IN BOOLEAN MODE) AS Relevance, NULL AS CanViewType FROM `Product_live` WHERE (MATCH (Content,TechnicalInformation) AGAINST ('something*') AND ClassName = 'Product') AND (`SiteTree_Live`.ClassName IN ('Product')) ORDER BY Relevance DESC LIMIT 0, 10 Unknown column 'SiteTree_Live.ClassName' in 'where clause'