....
public function store($id){
        $cek = \DB::table('m_buku')->where('id',$id)->where('stock','>',0)->where('status',1)->count();

        if($cek > 0){
            \DB::table('peminjaman')->insert([
                'buku'=>$id,
                'user'=>\Auth::user()->id,
                'created_at'=>date('Y-m-d H:i:s')
                ]);

            $buku = \DB::table('m_buku')->where('id',$id)->first();
            $qty_now = $buku->stock;
            $qty_new = $qty_now - 1;

            \DB::table('m_buku')->where('id',$id)->update([
                'stock'=>$qty_new
            ]);

            \Session::flash('sukses','buku berhasil di pinjam');

            return redirect('master/buku');
        }else{
            \Session::flash('gagal','buku sudah habis atau tidak aktif');

            return redirect('master/buku');
        }
    }
....