Yii2 sort object defaultOrder - yii2

Question

I have a table of policies that belong to chapters. When I view the policies in my grid view I want the default order be policy title within chapter title. I see how to set up sort attributes to enable this, but I can't figure out how to set the defaultOrder to be based on chapter title and then policy title. When ever I try to set policy.title as an attribute in the defaultOrder setting I get an error.

Answer 1

If Policy is model class of policy table that has a relation with chapter table named 'chapter' and linked by chapter_id field, such as:
public function getChapter()
{
return $this->hasOne(Chapter::className(), ['chapter_id' => 'chapter_id']);
}
Now you build query object with policy joined with chapter:
$query = Policy::find()
->joinWith(['chapter']);
$dataProvider = new ActiveDataProvider([
'query' => $query,
'sort'=> ['defaultOrder' => ['chapter.title'=>SORT_ASC, 'policy.title' => SORT_ASC]]
]);

Answer 2

I could not get Fabrizio's answer to work, because my chapter table is policy_chapter, not just chapter and so is not the same as the relation name. When I tried to use the relation name, I got errors. I finally figured out in the sort, you have to use the names of the related tables, not the relations, and add them as attributes. e.g.:
$dataProvider = new ActiveDataProvider([
'query' => $query,
'pagination' => ['pageSize' => 50],
'sort' => [
'defaultOrder' => ['policy_chapter.sort' => SORT_ASC,'policy_chapter.title' => SORT_ASC],
'enableMultiSort' => true,
'attributes' => [
'id' => [
'asc' => ['policy.id' => SORT_ASC],
'desc' => ['policy.id' => SORT_DESC],
'default' => SORT_ASC
],
'chapter_id' => [
'asc' => ['policy_chapter.sort' => SORT_ASC,'policy_chapter.title' => SORT_ASC],
'desc' => ['policy_chapter.sort' => SORT_DESC,'policy_chapter.title' => SORT_DESC],
'default' => SORT_ASC,
],
'reference' => [
'asc' => ['reference' => SORT_ASC],
'desc' => ['reference' => SORT_DESC],
'default' => SORT_ASC
],
'title' => [
'asc' => ['policy.title' => SORT_ASC],
'desc' => ['policy.title' => SORT_DESC],
'default' => SORT_ASC
],
'policy_chapter.sort',
'policy_chapter.title',
],
],
]);
Use the relation name or leave out the attributes and you get errors. Before I was writing
'defaultOrder' => ['policy_chapter.sort' => SORT_ASC,'policy_chapter.title' => SORT_ASC],
and Yii was not happy