WhereHas Laravel 中的关系计数条件是什么

我很难理解WhereHas的关系计数情况.文档页面上没有关于它的讨论，但API page谈到了它.引用API.

Builder|Builder whereHas(string $relation, Closure $callback, string $operator = '>=', int $count = 1)

Add a relationship count condition to the query with where clauses.

Example

Resource型号与ResourceCategory有多对多的关系

public function categories()
{
    return $this->belongsToMany('ResourceCategory', 'resource_category_mapping');
}

Relationship condition in Has

The relationship condition in Has is working as expected.

Resource::has('categories', '>', 1)->get()
//this return all resources which have more than one catgories

Relationship condition in WhereHas

The relationship condition in WhereHas is not working as expected. I am sure I have misunderstood it.

Resource::whereHas('categories', function ( $query){
            $query->whereIn('resource_category_id', [1, 2, 4]);
        }, '>', 1)->get()

The above code should return resources which whose categories belong to either of [1, 2, 4] and the resource has more than one categories. But it is not.

Question

Kindly explain the relationship condition in WhereHas, may be providing an example would be much helpful.