Self Referencing Relationship?

Hi all,

Thanks in advance for any help & advice.

I'm building an API and one endpoint is to return information about a "call" (job) to a user with the relationships "appointments" and "call_actions".

If "call_actions" doesn't exist, I want to attempt to find the previous "call" and get it's call_actions relationship so the consumer can use it to pre-fill a form.

Unfortunately whenever I try I get "InvalidArgumentException".

Could anyone please advise/point me in the right direction?

Thank you!

Model (SCCall.php)

<?php

namespace App\Models\Tesseract;

use App\Enums\CallStatus;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class SCCall extends Model
{
    use HasFactory;

    protected $connection = 'sqlsrv';

    protected $table = 'SCCall';

    protected $primaryKey = 'Call_Num';

    public $timestamps = false;

    public function appointment()
    {
        return $this->hasOneThrough(
            SCDiaryEntry::class,
            SCDiaryEntryLinkToCall::class,
            'Call_Num',           // Foreign key on SCDiaryEntryLinkToCall
            'DiaryEntry_ID',      // Foreign key on SCDiaryEntry
            'Call_Num',           // Local key on SCCall
            'DiaryEntry_ID'       // Local key on SCDiaryEntryLinkToCall
        )
        ->orderBy('Appointment_Date', 'desc');
    }

    public function latestPreviousPPMCall()
    {
        return $this->belongsTo(SCCall::class)
            ->where('Call_CalT_Code', 'PPM')
            ->where('Call_Status', 'COMP')
            ->where('Call_Prod_Num', $this->Call_Prod_Num)
            ->where('Call_Site_Num', $this->Call_Site_Num)
            ->where('Call_Prob_Code', $this->Call_Prob_Code)
            ->where('Call_Num', '<', $this->Call_Num)
            ->orderBy('Call_Num', 'desc');
    }

Controller

<?php

namespace App\Http\Controllers\API;

use App\Models\Tesseract\SCCall;
use App\Http\Controllers\Controller;
use Illuminate\Http\Request;
use Illuminate\Http\JsonResponse;

class CallsController extends Controller
{
    public function show($call_num)
    {
        $call = SCCall::with([
            'appointment',
            'call_actions'
        ])
        ->where('Call_Num', $call_num)
        ->first();

        if (!$call) {
            return new JsonResponse(['message' => 'Call not found'], 404);
        }

        // If no call_actions exist, try to get them from the previous completed call
        if (!$call->call_actions) {
            $call->load('latestPreviousPPMCall.call_actions');

            if ($call->latestPreviousPPMCall && $call->latestPreviousPPMCall->call_actions) {
                $call->call_actions = $call->latestPreviousPPMCall->call_actions;
            }
        }

        // Set default values if still no call_actions found
        if (!$call->call_actions) {
            $call->call_actions = new \stdClass();
            $call->call_actions->Email_To_Customer = false;
            $call->call_actions->Remedial_Required = false;
            $call->call_actions->Assets_On_Site = false;
            $call->call_actions->FSR = '';
            $call->call_actions->Remedial_Works = '';
            $call->call_actions->System_Info = [];
        }

        return new JsonResponse($call, 200);
    }