Yii2: Sorting by calculated column - yii2

Question

I have one table like:
name
startDate
endDate
units
price
and I show them in a gridview for a given year as follows (index.php in views):
$gridColumns = [
'name',
[
'attribute' => 'january',
'format' => 'currency',
'value' => function ($data) {
if(date("m", strtotime($data["startDate"])) == 1) {
return ($data["price"]*$data["units"]);
}
else {
return "";
}
},
'options' => ['style' => 'max-width:20px;'],
'pageSummary' => true,
],
[
'attribute' => 'february',
'format' => 'currency',
'value' => function ($data) {
if(date("m", strtotime($data["startDate"])) == 2) {
return ($data["price"]*$data["units"]);
}
else {
return "";
}
},
'pageSummary' => true,
],
... until month 12
];
In my search model i have defined:
public $january, $february, $march, $april, $may, $june, $july, $august, $september, $october, $november, $december;
placed them as safe in rules and defining attributes in order to get grid header with sortable items
$dataProvider = new ActiveDataProvider([
'query' => $query,
'pagination' => [
'pageSize' => 0,
],
'sort' => [
'defaultOrder' => [
'name' => SORT_ASC,
],
'attributes' => [
'name',
'units',
'price',
'january',
'february',
'march',
'april',
'may',
'june',
'july',
'august',
'september',
'october',
'november',
'december',
],
],
]);
but when i try to sort by one of the months it gives an error like:
exception 'PDOException' with message 'SQLSTATE[42S22]: Column not
found: 1054 Unknown column 'january' in 'order clause''
How could I sort by these calculated columns that doesnt exist in the database?
Thanks