Implement parent child ticket hierarchy

app/Http/Requests/UpdateProjectTicketRequest.php

@@ -3,10 +3,13 @@
 namespace App\Http\Requests;
 
 use App\Models\Project;
+use App\Models\Ticket;
+use App\Models\TicketType;
 use Illuminate\Contracts\Validation\ValidationRule;
 use Illuminate\Foundation\Http\FormRequest;
 use Illuminate\Support\Arr;
 use Illuminate\Validation\Rule;
+use Illuminate\Validation\Validator;
 
 class UpdateProjectTicketRequest extends FormRequest
 {
@@ -40,6 +43,11 @@ class UpdateProjectTicketRequest extends FormRequest
             ],
             'field_values.*.value_number' => ['nullable', 'numeric'],
             'field_values.*.value_user_id' => ['nullable', 'integer', Rule::exists('users', 'id')],
+            'parent_ticket_id' => [
+                'nullable',
+                'integer',
+                Rule::exists('tickets', 'id')->where('project_id', $project->id),
+            ],
             'ticket_priority_id' => [
                 'required',
                 'integer',
@@ -54,10 +62,69 @@ class UpdateProjectTicketRequest extends FormRequest
         ];
     }
 
+    /**
+     * @return array<int, callable(Validator): void>
+     */
+    public function after(): array
+    {
+        return [
+            function (Validator $validator): void {
+                /** @var Project $project */
+                $project = $this->route('project');
+
+                /** @var Ticket $ticket */
+                $ticket = $this->route('ticket');
+
+                $parentTicketId = $this->input('parent_ticket_id');
+                $parentTicketId = is_numeric($parentTicketId) ? (int) $parentTicketId : null;
+
+                if ($parentTicketId !== null) {
+                    if ($parentTicketId === $ticket->id) {
+                        $validator->errors()->add('parent_ticket_id', 'A ticket cannot be its own parent.');
+
+                        return;
+                    }
+
+                    $parentTicket = Ticket::query()
+                        ->where('project_id', $project->id)
+                        ->with('ticketType')
+                        ->find($parentTicketId);
+
+                    if ($parentTicket !== null && $parentTicket->ticketType?->system_key !== 'parent') {
+                        $validator->errors()->add('parent_ticket_id', 'Only tickets of type Parent can be selected as a parent.');
+
+                        return;
+                    }
+
+                    if ($parentTicket !== null && $this->createsCycle($ticket, $parentTicket)) {
+                        $validator->errors()->add('parent_ticket_id', 'A ticket cannot be assigned to one of its descendants.');
+
+                        return;
+                    }
+                }
+
+                $requestedTicketTypeId = $this->input('ticket_type_id');
+                $requestedTicketType = is_numeric($requestedTicketTypeId)
+                    ? TicketType::query()
+                        ->where('project_id', $project->id)
+                        ->find((int) $requestedTicketTypeId)
+                    : null;
+
+                if (
+                    $requestedTicketType?->system_key !== 'parent'
+                    && $ticket->childTickets()->exists()
+                ) {
+                    $validator->errors()->add('ticket_type_id', 'Tickets with child tickets must keep a Parent type.');
+                }
+            },
+        ];
+    }
+
     /**
      * @return array{
      *     acceptance_criteria: string|null,
      *     description: string|null,
+     *     parent_ticket_id: int|null,
      *     ticket_priority_id: int,
      *     ticket_type_id: int,
      *     title: string
@@ -68,18 +135,27 @@ class UpdateProjectTicketRequest extends FormRequest
         /** @var array{
          *     acceptance_criteria: string|null,
          *     description: string|null,
+         *     parent_ticket_id: int|null,
          *     ticket_priority_id: int,
          *     ticket_type_id: int,
          *     title: string
          * } $attributes
          */
-        $attributes = Arr::only($this->validated(), [
-            'acceptance_criteria',
-            'description',
-            'ticket_priority_id',
-            'ticket_type_id',
-            'title',
-        ]);
+        $attributes = array_merge(
+            [
+                'acceptance_criteria' => null,
+                'description' => null,
+                'parent_ticket_id' => null,
+            ],
+            Arr::only($this->validated(), [
+                'acceptance_criteria',
+                'description',
+                'parent_ticket_id',
+                'ticket_priority_id',
+                'ticket_type_id',
+                'title',
+            ]),
+        );
 
         return $attributes;
     }
@@ -103,4 +179,23 @@ class UpdateProjectTicketRequest extends FormRequest
 
         return $fieldValues;
     }
+
+    private function createsCycle(Ticket $ticket, Ticket $candidateParent): bool
+    {
+        $currentParent = $candidateParent;
+
+        while ($currentParent !== null) {
+            if ($currentParent->id === $ticket->id) {
+                return true;
+            }
+
+            if ($currentParent->parent_ticket_id === null) {
+                return false;
+            }
+
+            $currentParent = Ticket::query()->find($currentParent->parent_ticket_id);
+        }
+
+        return false;
+    }
 }