Many To Many json api in laravel 5.1

I make a category and subcategory with Many To Many relationship

and i want to display it with Json api to be like this

[
    {
        "id": "1",
        "title": "Main Category",
        "subCategories": [
        "id": "1",
        "title": "SubCategory",
        ]
    {
]

category Model

public function subcategory()
{
    return $this->belongsToMany('App\Subcategory', 'category_id');
}

Subcategory Model

public function category()
{
    return $this->belongsToMany('App\Categories', 'categories_subcategories');
}

via Chebli Mohamed