Exercise 1: Adding One-to-Many and One-to-One Relations — Possible Solution
=============================================================================

schema.prisma (relation parts only; keep your other fields):

  model User {
    id      Int      @id @default(autoincrement())
    email   String   @unique
    name    String?
    profile Profile?
    posts   Post[]
  }

  model Profile {
    id     Int     @id @default(autoincrement())
    bio    String?
    userId Int     @unique
    user   User    @relation(fields: [userId], references: [id], onDelete: Cascade)
  }

  model Post {
    id       Int    @id @default(autoincrement())
    title    String
    slug     String @unique
    authorId Int
    author   User   @relation(fields: [authorId], references: [id])

    @@index([authorId])
  }

  $ npx prisma migrate dev --name add-relations
    (existing posts have no author, so accept the reset in development —
     see the chapter's warning box)
  $ npx prisma generate

Creating the data using foreign keys:

  const user = await prisma.user.create({
    data: { email: "grace@example.com", name: "Grace" },
  });

  await prisma.profile.create({
    data: { bio: "Compiler pioneer", userId: user.id },
  });

  await prisma.post.createMany({
    data: [
      { title: "First",  slug: "first",  authorId: user.id },
      { title: "Second", slug: "second", authorId: user.id },
    ],
  });

Try creating a second profile with the same userId: it fails with a
P2002 unique constraint error, which is the one-to-one rule working.

WHY THIS WORKS AS AN ANSWER
------------------------------
It declares both sides of each relation, uses @unique for one-to-one,
indexes the posts' foreign key, and sets Cascade only on the profile,
as the exercise asked.