Do I have to provide a back reference? What if it's not there? For instance, consider an example:

model Inout {
  id             Int             @id @default(autoincrement())
  name           String
  inoutConfigSdi InoutConfigSdi?
  inoutConfigEth InoutConfigEth?
}

model InoutConfigSdi {
  id         Int    @id @default(autoincrement())
  portNumber String
  inout      Inout  @relation(fields: [inoutId], references: [id])
  inoutId    Int    @unique
}

model InoutConfigEth {
  id      Int    @id @default(autoincrement())
  address String
  inout   Inout  @relation(fields: [inoutId], references: [id])
  inoutId Int    @unique
}

Here

inoutConfigSdi InoutConfigSdi?
  inoutConfigEth InoutConfigEth?

look superfluous, as Input can have only ONE type of Config - either Sdi or Eth. But I don't see any way to declare it. Any ideas?