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.
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Wiki articles tagged with "count"
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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.