how to get relationship model's relationship_id using transformer store in array

I am using laravel 5.1 with transformer and json api. I had combined relationship with product model in category model. one product inserted as in many categories, so that category_id to array format. now how to relate product model's category_id using transformer with $this operater.

my category model:

`

public function products()
 {
 return $this->hasMany('App\Product','category_id');
 }
 public function product()
 {
 $category_id=json_decode($this->category_id);
 return Product::where('category_id',$this->category_id)->select('name','description','image','price','mrp','wsprice','quantity','attributes')->get();
}`

my transformer:

public function transform(Category $category){
return [
'id' => $category->category_id,
'name' => $category->name,
'slug'=>$category->slug,
'image'=>$category->image,
'product'=>$category->product(),
'subcategory'=>$category->subcategery()              
];
}`

via Chebli Mohamed