This adds the foundation for polymorphic relations (e.g., item Post | Comment):

• Add isPolymorphic, relationTypes, relationDiscriminator to DMMF types
• Generate union types for polymorphic relation outputs in TS/JS generators
• Add { on: 'Post' } syntax support for include/select filtering
• Handle discriminator-based query routing in runtime
• Add test fixture for polymorphic relations

Note: Schema parser changes in prisma-engines are still needed to parse the Post | Comment syntax and @polymorphic attribute.

Example Usage

###Schema

model Vote {
  id        String         @id @default(cuid())
  item      Post | Comment @polymorphic(discriminator: itemType)
  itemId    String
  itemType  String
}

model Post {
  id    String @id @default(cuid())
  title String
  votes Vote[]
}

model Comment {
  id   String @id @default(cuid())
  body String
  votes Vote[]
}

Querying

// Union type (Post | Comment)
const vote = await prisma.vote.findUnique({
  where: { id: '1' },
  include: { item: true },
})
// vote.item: Post | Comment

// Filtered by `on` (Post only)
const voteWithPost = await prisma.vote.findUnique({
  where: { id: '1' },
  include: { item: { on: 'Post' } },
})
// voteWithPost.item: Post | null

// Filter by discriminator
const postVotes = await prisma.vote.findMany({
  where: { itemType: 'Post' },
  include: { item: true },
})