Bagaimana membuat nested foreach multiple di codeigniter

Saya coba menampilkan produk berdasarkan kategori di dalam sub kategori, tapi sepertinya tidak menghasilkan output yang saya inginkan.

Struktur tabel nya seperti ini:

Testing kode nya:

function product() {
        $category = $this->db->query("select * from tb_product_category");

        foreach($category as $key) {
            $subcategory = $this->db->query("select * from tb_product_sub_category where id_product_category = '".$key->id_product_category."'");
            foreach($subcategory as $cat) {
                $product = $this->db->query("select * from tb_product where id_product_category = '".$key->id_product_category."'");
                $key->sub_category = array(
                    'sub_category_name' => $cat
                );
                foreach($product as $prod) {
                    $key->product = array(
                    'product' => $prod
                );
                }
            }
        }

        echo json_encode($category);
    }

Saya ingin hasil nya seperti dibawah ini:

{
    "data": [
        {
            "id_product_category": "1",
            "category_name": "Sembako",
            "sub_category": [
                {
                    "sub_category_name": "Beras",
                    "product": [
                        {
                            "id_product": "3",
                            "product_name: "BerasKita Premium 5 Kg",
                            "price": "65000",
                        }
                    ]
                },
                {
                    "sub_category_name": "Gula",
                    "product": [
                        {
                            "id_product": "5",
                            "product_name: "Gula ManisKita 1 Kg",
                            "price": "12500",
                        }
                    ]
                },
                {
                    "sub_category_name": "Minyak",
                    "product": [
                        {
                            "id_product": "1",
                            "product_name: "Minyak Goreng Vipco 1 lt",
                            "price": "12500",
                        },
                        {
                            "id_product": "4",
                            "product_name: "Minyak GorengKita 1 Ltr",
                            "price": "12500",
                        }
                    ]
                },
                {
                    "sub_category_name": "Terigu",
                    "product": [
                        {
                            "id_product": "7",
                            "product_name: "Tepung Ketan Rose Brand",
                            "price": "10500",
                        }
                    ]
                }
            ],
        },
        {
            "id_product_category": "2",
            "category_name": "Bumbu",
            "sub_category": [
                {
                    "sub_category_name": "Masakan",
                    "product": [
                        {
                            "id_product": "24",
                            "product_name: "Garam Dapur Himalayan Salt",
                            "price": "19500",
                        }
                    ]
                },
                {
                    "sub_category_name": "Kecap",
                    "product": [
                        {
                            "id_product": "23",
                            "product_name: "Bango Kecap Manis Refill 550 ml",
                            "price": "23000",
                        }
                    ]
                }
            ],
        },
    ]
}

Mohon bantuan nya..