Array to string conversion in Laravel 11 Models

I am having a problem with my relationships in model using composite key. I am using the package ThiagoprzCompositeKeyHasCompositeKey. If i am only using a single key for the relationships in models, I am not having a problem. I also wanna ask too if what is the correct usage for the controllers. Here is my current controller

Here is so far what I got from debugging From PHP Tinker

public function index(Request $request)
    {
        try {
            $crse_id = $request->input('crse_id');
            $crse_offer_nbr = $request->input('crse_offer_nbr');
            $strm = $request->input('strm');
            $session_code = $request->input('session_code');
            $class_section = $request->input('class_section');

            $classData = ClassTbl::with([
                'classChrstc',
                'institution:institution,descr', 
                'termTable:strm,descr',         
                'academicSubject:subject'     
            ])
            ->where('crse_id', $crse_id)
            ->where('crse_offer_nbr', $crse_offer_nbr)
            ->where('strm', $strm)
            ->where('session_code', $session_code)
            ->where('class_section', $class_section)
            ->first();

            if (!$classData) {
                return response()->json([
                    'success' => false,
                    'message' => 'Class data not found.'
                ], 404);
            }

            return response()->json([
                'success' => true,
                'data' => $classData,
                'message' => 'Class data retrieved successfully.'
            ]);

        } catch (Exception $e) {
            return response()->json([
                'success' => false,
                'message' => 'Failed to retrieve class data.',
                'error' => $e->getMessage()
            ], 500);
        }
    }

    public function classTbl()
    {
        return $this->belongsTo(ClassTbl::class, 'crse_id', 'crse_id')
            ->whereColumn('class_tbl.crse_offer_nbr', 'class_chrstcs.crse_offer_nbr')
            ->whereColumn('class_tbl.strm', 'class_chrstcs.strm')
            ->whereColumn('class_tbl.session_code', 'class_chrstcs.session_code')
            ->whereColumn('class_tbl.class_section', 'class_chrstcs.class_section');
    }

    public function classChrstc()
    {
        return $this->hasMany(ClassChrstc::class, 'crse_id', 'crse_id')
            ->whereColumn('class_chrstcs.crse_offer_nbr', 'class_tbl.crse_offer_nbr')
            ->whereColumn('class_chrstcs.strm', 'class_tbl.strm')
            ->whereColumn('class_chrstcs.session_code', 'class_tbl.session_code')
            ->whereColumn('class_chrstcs.class_section', 'class_tbl.class_section');
    }