prisma nested writes / updates not working

generator client {
  provider = "prisma-client-js"
}

datasource db {
  provider = "mysql"
  url = env("DATABASE_URL")
  relationMode = "prisma"
}

model Exams {
  id      Int   @default(autoincrement()) @id
  name    String
  file    String
  subject String?
  year    Int
}

model Subjects {
  id      Int      @default(autoincrement()) @id
  name    String
  topics  Topic[]
}

model Topic {
  id         Int      @default(autoincrement()) @id
  name       String
  subjectId  Int
  subject    Subjects @relation(fields: [subjectId], references: [id])
  @@index([subjectId])
}

import { db } from "../../lib/db"
import type { NextApiRequest, NextApiResponse } from 'next';
import { SubjectsWhereUniqueInput } from './types';

export async function POST(
  req: NextApiRequest,
  res: NextApiResponse
) {
  try {
    const { name, subject } = req.body;
    const subjectRecord = await db.subjects.findUnique({
      where: { name: subject } as SubjectsWhereUniqueInput,
    });

    if (!subjectRecord) {
      return res.status(404).json({ message: 'Subject not found' });
    }

    const topic = await db.topic.create({
      data: {
        name,
        subject: { connect: { id: subjectRecord.id } },
      },
    });
    res.status(201).json(topic);
  } catch (error) {
    console.error(error);
    res.status(500).json({ message: 'Internal Server Error' });
  }
}