https://github.com/owen-it/laravel-auditing/issues/58

changes.md

• we call the savePivotAudit directly to avoid errors of unknown model attributes

• using get_class($model->$relationName()->getRelated()) to avoid error of null value

• check if $pivotIds is avail b4 saving or we might get an error "edge case"

• removed all the extra attributes like user, tags, url, ip, etc.. as they can be fetched from the main audit record

create_audits_pivot_table.php

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateAuditsPivotTable extends Migration
{
    /**
     * Run the migrations.
     */
    public function up()
    {
        Schema::create('audits_pivot', function (Blueprint $table) {
            $table->increments('id');
            $table->string('event');
            $table->morphs('auditable');
            $table->morphs('relation');
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     */
    public function down()
    {
        Schema::dropIfExists('audits_pivot');
    }
}

MyAuditable.php

<?php

namespace App\Traits;

use Fico7489\Laravel\Pivot\Traits\PivotEventTrait;
use OwenIt\Auditing\Auditable;

/**
 * https://gist.github.com/AbbyJanke/4d245b22dbcec277c207f033f37dae3b.
 */
trait MyAuditable
{
    use Auditable, PivotEventTrait;

    public static function bootMyAuditable()
    {
        static::pivotAttaching(function ($model, $relationName, $pivotIds, $pivotIdsAttributes) {});
        
        static::pivotAttached(function ($model, $relationName, $pivotIds, $pivotIdsAttributes) {
            if ($pivotIds) {
                return $model->savePivotAudit(
                    'Attached',
                    get_class($model->$relationName()->getRelated()),
                    $pivotIds[0],
                    $model->getKey()
                );
            }
        });
        
        static::pivotDetaching(function ($model, $relationName, $pivotIds) {});
        
        static::pivotDetached(function ($model, $relationName, $pivotIds) {
            if ($pivotIds) {
                return $model->savePivotAudit(
                    'Detached',
                    get_class($model->$relationName()->getRelated()),
                    $pivotIds[0],
                    $model->getKey()
                );
            }
        });
        
        static::pivotUpdating(function ($model, $relationName, $pivotIds, $pivotIdsAttributes) {});
        
        static::pivotUpdated(function ($model, $relationName, $pivotIds, $pivotIdsAttributes) {});
    }

    private function savePivotAudit($eventName, $relationClass, $relationId, $modelId)
    {
        return app('db')->table('audits_pivot')->insert([
                'event'          => $eventName,
                'auditable_id'   => $modelId,
                'auditable_type' => $this->getMorphClass(),
                'relation_type'  => $relationClass,
                'relation_id'    => $relationId,
                'created_at'     => now(),
                'updated_at'     => now(),
            ]);
    }

    /**
     * normal : $model->audits
     */
    private function getPivotAudits($type, $id)
    {
        return app('db')->table('audits_pivot')
                ->where('auditable_id', $id)
                ->where('auditable_type', $type)
                ->get()
                ->reverse();
    }

    /**
     * with relation : $model->auditsWithRelation
     */
    public function getAuditsWithRelationAttribute()
    {
        return $this->audits->map(function ($item) {
            $item['relations'] = $this->getPivotAudits($item->auditable_type, $item->auditable_id);

            return $item;
        });
    }
}

For anyone coming across this and wondering if it will work in combination with Nova. It doesn't.