How do I save an activity to a user's activity array using Prisma for MongoDB and Next.js 14 server actions?

Question

I am able to successfully create a user and an activity to my Mongo database. However, the activity is not showing up in the user's activity array.

Here is my Prisma schema:

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

datasource db {
  provider = "mongodb"
  url      = env("DATABASE_URL")
}

model User {
  id    String @id @default(auto()) @map("_id") @db.ObjectId
  email String @unique
  name  String?
  avatar String? @default("")
  createdAt DateTime @default(now())
  updatedAt DateTime @updatedAt
  activities Activity[] 
  @@map("users")
}

model Activity {
  id    String @id @default(auto()) @map("_id") @db.ObjectId
  // date DateTime
  type String
  distance String
  time String
  region String
  description String?
  user User @relation(fields: [userId], references: [id])
  userId String @db.ObjectId
  @@map("activities")
}

Here is my Next.js 14 server action function where I successfully create an activity and try to then update the user's activity array to include the newly created activity:

export const createActivityAction = async (
  type: string,
  distance: string,
  time: string,
  region: string,
  userId: string,
  description: string
) => {
  try {
    const activity = await prisma.activity.create({
      data: {
        type,
        distance,
        time,
        region,
        description,
        user: {
          connect: {
            id: userId,
          },
        },
      },
    });

    const user = await prisma.user.update({
      where: {
        id: userId,
      },
      data: {
        activities: {
          connect: {
            id: activity.id,
          },
        },
      },
    });
    revalidatePath("/create-activity");
    return { activity, user };
  } catch (error) {
    return { message: "Database Error:Failed to Create Post" };
  }
};

I am expecting the activity to show up in the user's activity array so that I can then map over the user's activities on the "my activities" page.