How to fix P2015: A related record could not be found in Prisma

PrismaClientKnownRequestError:
P2015: A related record could not be found.
Details: Foreign key constraint failed on the field: `Post_userId_fkey`

// Example: Check if user exists before creating post
const userId = 'some-user-id';
const userExists = await prisma.user.findUnique({
  where: { id: userId }
});

if (!userExists) {
  throw new Error(`User with ID ${userId} does not exist`);
}

// Now create the post
await prisma.post.create({
  data: {
    title: 'My Post',
    author: {
      connect: { id: userId }
    }
  }
});

-- For PostgreSQL/MySQL
SELECT id FROM User WHERE id = 'some-user-id';

await prisma.post.create({
  data: {
    title: 'My Post',
    author: {
      connectOrCreate: {
        where: { id: userId },
        create: {
          id: userId,
          name: 'New User',
          email: '[email protected]'
        }
      }
    }
  }
});

await prisma.$transaction(async (tx) => {
  // Delete user and all related posts in one transaction
  await tx.post.deleteMany({
    where: { userId: userIdToDelete }
  });

  await tx.user.delete({
    where: { id: userIdToDelete }
  });
});

await prisma.$transaction(async (tx) => {
  const user = await tx.user.findUnique({
    where: { id: userId }
  });

  if (!user) {
    throw new Error('User not found');
  }

  await tx.post.create({
    data: {
      title: 'My Post',
      author: {
        connect: { id: userId }
      }
    }
  });
});

function isValidId(id: string): boolean {
  // Check if ID matches expected format
  return /^[a-zA-Z0-9_-]{21}$/.test(id); // Example for cuid()
}

async function createPostWithValidation(postData: any) {
  if (!isValidId(postData.userId)) {
    throw new Error('Invalid user ID format');
  }

  // Additional validation...
  return await prisma.post.create({
    data: {
      title: postData.title,
      author: {
        connect: { id: postData.userId }
      }
    }
  });
}

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

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

model Post {
  id       String  @id @default(cuid())
  title    String
  userId   String?  // Nullable for optional relation
  user     User?    @relation(fields: [userId], references: [id])
}