Nested JSON show wrong values PHP MySQL - php

Question

I want to create a JSON format from 3 mysql tables using PHP. Below is my table:
To create a JSON format from these 3 tables I am using this code:
<?php
ini_set('display_errors', 'Off');
$user = "root";
$password = "";
$database = "mydb";
$conn = mysqli_connect("localhost", $user, $password, $database) or die ("Unable to connect");
$sql = "SELECT model_id FROM 1Model";
$sqlRun = mysqli_query($conn , $sql);
$json = array();
$role_model = mysqli_num_rows($sqlRun);
if($role_model > 0){
$i = 0;
while($row = mysqli_fetch_array($sqlRun)){
$row_array= array();
$model_id = $row['model_id'];
$json[$i]['model_id'] = $row['model_id'];
$role_qry = mysqli_query($conn, "SELECT id_cat, cat_name, model_id FROM 1Category WHERE model_id= $model_id");
while($role_fet = mysqli_fetch_array($role_qry)){
$json[$i]['id_cat']['id_cat'] = $role_fet['id_cat'];
$json[$i]['cat_name']['cat_name'] = $role_fet['cat_name'];
$cat_array = array();
$cat_pk = $role_fet['id_cat'];
$test_query = mysqli_query($conn, "SELECT id_item, item_name, id_cat FROM 1Item WHERE id_cat = $cat_pk");
$j = 0;
while($test_fet = mysqli_fetch_array($test_query)){
$json[$i]['item_list']['items'][$j] = array('item_name' => $test_fet['item_name']);
$j++;
}
}
$i++;
}
}
header('Content-type: text/javascript');
echo json_encode($json, JSON_PRETTY_PRINT);
?>
My expected result is json object will filter based on the model name and category name, something like below:
[
{
"model_id": "1",
"cat_name": {
"cat_name": "Box Medium"
"item_list": {
"items": [
{
"item_name": "Box Medium Orange"
},
{
"item_name": "Box Medium Purple"
},
{
"item_name": "Box Medium White"
},
{
"item_name": "Box Medium Grey"
}
]
}
},
},
{
"model_id": "2",
"id_cat": {
"id_cat": "6"
},
"cat_name": {
"cat_name": "Square Large"
}
},
{
"model_id": "3"
}
]
But I don't know why my result children values not filtered based on parent id.
Right now the code encode json as result below:
[
{
"model_id": "1",
"id_cat": {
"id_cat": "3"
},
"cat_name": {
"cat_name": "Box Large"
},
"item_list": {
"items": [
{
"item_name": "Box Medium Orange"
},
{
"item_name": "Box Medium Purple"
},
{
"item_name": "Box Medium White"
},
{
"item_name": "Box Medium Grey"
}
]
}
},
{
"model_id": "2",
"id_cat": {
"id_cat": "6"
},
"cat_name": {
"cat_name": "Square Large"
}
},
{
"model_id": "3"
}
]
Anyone can help me on this, tried to find on the web but unlucky. What I am trying to do is to get a json format hierarchical, model=parent, category=subparent, item=child. Thanks for any helps