@unique is added during Re-Introspection

[janpio]

Original migrated schema:

// This is your Prisma schema file,
// learn more about it in the docs: https://pris.ly/d/prisma-schema

generator client {
  provider        = "prisma-client-js"
  previewFeatures = ["referentialIntegrity"]
}

datasource db {
  provider             = "mysql"
  url                  = env("DATABASE_URL")
  referentialIntegrity = "prisma"
}

model Foo {
  id     Int @id
  bar    Bar @relation(fields: [bar_id], references: [id])
  bar_id Int
}

model Bar {
  id  Int  @id
  foo Foo?
}

Re-Introspection then adds a @unique:

generator client {
  provider        = "prisma-client-js"
  previewFeatures = ["referentialIntegrity"]
}

datasource db {
  provider             = "mysql"
  url                  = env("DATABASE_URL")
  referentialIntegrity = "prisma"
}

model Foo {
  id     Int @id
  bar    Bar @relation(fields: [bar_id], references: [id])
  bar_id Int @unique
}

model Bar {
  id  Int  @id
  foo Foo?
}

(Most probably an expected feature, but still a bit surprising)

[pantharshit00]

I can confirm this. Looks like prisma-fmt also adds this @unique when you reformat the schema to ensure a 1:1 relation.

[janpio]

Ah ok, then it could be a side effect of that behavior.

[do4gr]

This is expected. We create a unique constraint in the database to enforce that the relation is 1:1 but currently do not require the @unique to be present in the datamodel. During reformatting or re-introspection we make it explicit though. We want to require the @unique to be explicit, but since this is a major breaking change it's a topic for Prisma 4.

[janpio]

Issue for that work: #10503