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.
Yii Framework Wiki
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Filter & Sort by Summary Data in GridView Yii 2.0
This wiki explains how to add summary data from related models into your Yii Framework 2.0 gridview with filtering and sorting by the summary columns.
Searching and Sorting by Count of Related Items in CGridView
class Post extends CActiveRecord {
...
}
`
When you list Authors in grid you would like to print the count of posts in column, allow sorting on this column and probably filtering by the count.