How to fix P2014: The change you are trying to make would violate the required relation '{relation_name}' between the {model_a_name} and {model_b_name} models. in Prisma

npx prisma --version

// Before deleting a user, delete their posts
await prisma.post.deleteMany({
  where: { userId: user.id }
});

// Now you can safely delete the user
await prisma.user.delete({
  where: { id: user.id }
});

await prisma.$transaction([
  prisma.post.deleteMany({ where: { userId: user.id } }),
  prisma.user.delete({ where: { id: user.id } })
]);

model User {
  id    String @id @default(cuid())
  posts Post[]
}

model Post {
  id     String @id @default(cuid())
  userId String
  user   User   @relation(fields: [userId], references: [id], onDelete: Cascade)
}

npx prisma migrate dev --name add-cascade-delete

// Instead of this (causes P2014 on update):
await prisma.post.upsert({
  where: { id: postId },
  update: {
    title: 'New Title',
    user: { connect: { id: userId } } // ❌ Already connected
  },
  create: {
    title: 'New Title',
    user: { connect: { id: userId } }
  }
});

// Do this (only connect on create):
await prisma.post.upsert({
  where: { id: postId },
  update: {
    title: 'New Title'
    // ✅ Don't reconnect what's already connected
  },
  create: {
    title: 'New Title',
    user: { connect: { id: userId } }
  }
});

npm install prisma@latest @prisma/client@latest
npx prisma generate

model Post {
  id     String @id
  userId String
  user   User   @relation(fields: [userId], references: [id], onDelete: Cascade)
  //                                                           ^^^^^^^^^^^^^^^^
  //     Options: Cascade, Restrict, NoAction, SetNull, SetDefault
}

npx prisma migrate dev --name add-referential-actions