What is the correct way to write with with aggregate functions?

Question

ORM

There are 2 tables: products and reviews .
Products store information about a product, and reviews store reviews about it.
And there is a relationship:

public function rev()
    {
       return $this -> hasMany( Review::class )-> whereVerify(1);
    }

it works, everything is fine, but, I need to count the number of reviews itself.
When I do it like this

public function rev_count()
    {
       return $this -> hasMany( Review::class )-> whereVerify(1) -> count();
    }

and output:

$product = Product::whereId(1)->first();

echo $product -> rev_count();

everything works, but I need to return the number of reviews in this way:

$product = Product::whereId(1) -> with('rev_count')->first() -> toJson();

last one throws a bug

[Symfony\Component\Debug\Exception\FatalThrowableError]     
  Call to a member function addEagerConstraints() on integer

Answer 1

https://laravel.com/docs/5.4/eloquent-serialization...

Answer 2

https://laravel.com/docs/5.4/eloquent-relationships , section Counting Related Models