There are a few issues with the other solutions I originally used that I found from other wikis. I address the issues I had in this much simpler and shorter way. I am also going to explain what is going into way more detail than others to help people understand what's going on.
When one needs to select a record in a table that do not have related records through a relation, it is generally suggested to use a subquery.
Personnaly, I prefer a high level approach, and I have a method to do it with scopes.
Finding good looking, working and not abandoned gallery manager for Yii 1.x application is a tough task. Out of all I found, only yii-gallery-manager extension looked promising. But, after downloading it, it turned out, that there are certain unusual things to do, to include it in your application. So, I decided to write this article to remember all these for future reference.
If you develop rather complex applications with Yii2, you might already be familiar with yii2-app-advanced application template. Well, the template is awesome and provides high flexibility regarding code/configuration sharing among parts of an application.