Recursive relationship and nested eager loading with constraints - laravel

Question

I'm tryng to create nested eager loading with a where constraint on a recursive relationship
Models and query simulation:
Model Hierarchy
<?php
namespace App\Models;
use Illuminate\Database\Eloquent\Model;
class Hierarchy extends Model
{
protected $table = 'hierarchy';
protected $primaryKey = 'id_hierarchy';
protected $fillable = [
'name',
'parent_id'
];
/**
* #return HasMany
*
* This method implement recursive relationship
*/
public function children()
{
return $this->hasMany(Hierarchy::class, 'parent_id')->with('children');
}
/**
* #return HasMany
*/
public function grandchildren()
{
return $this->hasMany(Grandchild::class, 'id_hierarchy');
}
}
Model Grandchild
<?php
namespace App\Models;
use Illuminate\Database\Eloquent\Model;
class Grandchild extends Model
{
protected $table = 'grandchildren';
protected $primaryKey = 'id';
protected $fillable = [
'id_hierarchy',
'id_something'
'name'
];
/**
* #return BelongsTo
*/
public function hierarchy()
{
return $this->belongsTo(Hierarchy::class, 'id_hierarchy');
}
}
The following query does not return the grandchildren as it was supposed to;
public function read($id)
{
$data = Hierarchy::query()
->whereNull('parent_id')
->with(['children.grandchildren' => function ($query) use($id) {
$query->where('id_something', $id);
}])
->get();
}
The problem is in the constrain, because with the following query it returns the grandchildrren (although not filtered because it doesn't have the where condition)
$data = Hierarchy::query()
->whereNull('parent_id')
->with(['children.grandchildren'])
->get();
Thanks in advance for suggestions to resolve this issue.
Edited:
As the code is a simulation of the real case, I added 'id_something' to be clearer what is involved.
'id_something' is related to another model that is not represented here

Answer 1

Assuming that HomeCity is one of the related models for GrandChild and the relationship is defined as
//GrandChild.php
public function home_city()
{
return $this->hasMany(HomeCity::class);
}
Then the query to return GrandChild records who live in HomeCity (id_something is a column on home_city table) identified by $id may written as:
public function read($id)
{
$data = Hierarchy::query()
->whereNull('parent_id')
->with(['children' => function ($query) use($id) {
$query->with(['grandchildren' => function($query) use($id) {
$query->whereHas('home_city', fn($query) => $query->where('id_something', $id);
}]);
}])
->get();
}