how to track changes of relation between two models in yii2 - yii2

Question

I have two models with a simple 1-n relation (category and item with category_id). I would like to show how much items there is in the category. I would like to have the number cached rather then always doing the count, so I have an extra field in category table called "total_items_count".
How to best do this total count triggering, when to call the "countUpdate" function since the relation could change from several places (backend, api, frontend...).
My initial plan was to use AFTER_UPDATE event, but "link()" must be called after the item is stored in database (on adding new item at least) so then I do not know which category is the item related to. I also need to know the old category, in case item goes from one category to another. In the backend controller I am using $item->link('category', $categoryObj); as I might change this relation to n-n someday.
Any advice on how to have complete control if the link between item and category changes and then update the count for the old and new linked category?
Thanks

Answer 1

The first and most important thing you need to now, is that you need to update the category count field via the ActiveRecord::updateCounters() method, in order to prevent incorrect update from multiple places. So you can do one of those things:
In the category item model you can do the following:
public function save($runValidation = true, $attributeNames = null)
{
if (parent::save(runValidation, attributeNames)) {
$this->category->updateCounters(['total_items_count' => 1]);
return true;
}
return false;
}
Or you can write a centralized component (let's say app\components\Category) which you will call from the whole application. This component will have a method that saves an item. For example:
public function saveItem($category, $item)
{
if ($item->save()) {
$item->link('category', $category);
$category->updateCounters(['total_items_count' => 1]);
return true;
}
return false;
}
Of course, when you delete an item, you will call:
$category->updateCounters(['total_items_count' => -1]);
See the ActiveRecord::updateCounters() API
From the Yii2 documentation:
Note: If you use yii\db\ActiveRecord::save() to update a counter column, you may end up with inaccurate result, because it is likely the same counter is being saved by multiple requests which read and write the same counter value.
If you need to keep track of which is the old category, just create another column let's say "old_category" to the category item table and when you change the category of the item put the old category id there, and the new category id to the category_id column.
Does that helps? If you have any questions, please ask!