How to count the number of records using ActiveRecord in Yii2?

Question

Yii

For example, there is a table Book and Author (Authors). A book, 1 and there can be several openings for 1 book.

Book                    
id|  title|
Autor
id|book_id|name_auth

In addition, in the records, there are books whose authors have not yet been assigned, not added. That is, their id is not present in the Author table. I need to count the number of books that have authors.

$count=Book::find()
            ->leftJoin('Author', '`author`.`book_id` = `book`.`id`')
            ->count();

But this schema outputs the number of entries in the Author field. Only books are needed.

Answer 1

A book, 1 and there can be several openings for 1 book.

What the hell?
1. Why join here?
2.

$books = Book::find()
            ->with('author')
            ->all();
foreach($books as $book){
echo  "количество авторов у книги " . $book->title . ": " . count($book->author);
}

In this case, there should be a relationship with the name getAuthor in the Book model