在Laravel 5中通过ID和所有递归获取记录

I've two tables, looks that (migrations):

Schema::create('sets', function(Blueprint $table)
{
    $table->increments('id');

    $table->string('key');

    $table->string('name');

    $table->string('set_type');

    $table->integer('belongs_to')->unsigned();

    $table->timestamps();

    $table->foreign('belongs_to')->references('id')->on('sets')->onDelete('cascade');

});

Schema::create('posts', function(Blueprint $table)
{
    $table->bigIncrements('id');

    $table->bigInteger('user_id')->unsigned();

    $table->bigInteger('set_id')->unsigned();

    $table->string('post_type', 25);

    $table->text('post');

    $table->boolean('is_reported')->default(false);

    $table->boolean('is_hidden')->default(false);

    $table->timestamps();

    $table->foreign('user_id')->references('id')->on('users');
    $table->foreign('set_id')->references('id')->on('sets');

});

The 'set' table is for storing data in which the location (country, city...) the post should be view. For example, let's store some countries:

   id | key               | name        | belongs_to
   1  | europe            | Europe      | null
   2  | germany-all       | Germany     | 1
   3  | germany-berlin    | Berlin      | 2
   4  | germany-frankfurt | Frankfurt   | 2
   5  | poland-all        | Poland      | 1
   6  | poland-warsaw     | Warsaw      | 5
   7  | england-all       | England     | 1

而且，我的帖子把_id设置为6，从逻辑上看，当我想从欧洲(ID 1)获取帖子时，那个帖子也应该返回，因为6属于5，5属于1.这就是我想要做的.有可能在不使用太多PHP的情况下做到这一点吗？