Exercise 3: Designing a Comment Model — Possible Solution
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  model Comment {
    id        Int      @id @default(autoincrement())
    body      String
    createdAt DateTime @default(now())

    postId    Int
    post      Post     @relation(fields: [postId], references: [id], onDelete: Cascade)

    authorId  Int?
    author    User?    @relation(fields: [authorId], references: [id], onDelete: SetNull)

    @@index([postId])
    @@index([authorId])
  }

  // and add the other sides:
  model Post { ... comments Comment[] }
  model User { ... comments Comment[] }

Choices:

  post -> onDelete: Cascade
    A comment only makes sense under its post. If the post is deleted,
    its comments should go too; leaving them behind would create
    orphans nobody can see.

  author -> optional, onDelete: SetNull
    Guests can comment, so the relation must be optional anyway. If a
    user deletes their account, their comments stay, so the discussion
    still makes sense, but lose their link to the account (display them
    as "deleted user"). SetNull is also the default for optional
    relations; writing it out makes the intention clear.

Both foreign keys are indexed, because you'll list comments by post and
by author.

WHY THIS WORKS AS AN ANSWER
------------------------------
It chooses each referential action from what the data means, not
habit: comments depend on posts, but not on their authors' accounts.