Big problem , with Category and SubCategory models, please help!

Hello Guys, i have a problem to make Categories and SubCategories!

I want to get all products who belongs to SubCategory and if doesn't have product who belongs to sub to get all products from parent category i mean:

/category/{category} and if have subcategory in navbar /category/{category}/{sub}

Here is my code

Category.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;
use Cviebrock\EloquentSluggable\Sluggable;

class Category extends Model
{
    use Sluggable;
    protected $fillable = ['name','icon','subcategory_id','slug'];

    /**
     * @return array
     */
    public function sluggable(){
        return [
            'slug' => [
                'source' => 'name'
            ]
        ];
    }

    /**
     * @return string
     */
    public function getRouteKeyName(){
        return 'slug';
    }


    /**
     * @return \Illuminate\Database\Eloquent\Relations\HasMany
     */
    public function products() {
       return $this->belongsToMany(Product::class);
    }
}

SubCategory.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class SubCategory extends Model
{
    protected $fillable = ['name','slug'];

    /**
     * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
     */
    public function category() {
        return $this->belongsTo(Category::class);
    }
}

Product.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Product extends Model
{
    protected $fillable = [];

    /**
     * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
     */
    public function categories() {
       return $this->belongsToMany(Category::class);
    }
}

My Database structure:

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateCategoriesTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('categories', function (Blueprint $table) {
            $table->increments('id');
            $table->integer('parent_id')->nullable()->default(null);
            $table->string('name');
            $table->string('slug');
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('categories');
    }
}

SubCategory:

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateSubcategoriesTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('subcategories', function (Blueprint $table) {
            $table->increments('id');
            $table->string('name');
            $table->string('slug')->unique();
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('subcategories');
    }
}

Products:

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateProductsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('products', function (Blueprint $table) {
            $table->increments('id');
            $table->string('name');
            $table->text('description');
            $table->double('price');
            $table->text('image');
            $table->text('onlyOnce');
            $table->integer('discount')->nullable(true);
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('products');
    }
}

Please help thanks!