MongoDB connector: Model discriminators / single table inheritence

Problem

A common use case is to store different shapes of a document in the same collection and use a discriminator column to tell them apart. Raw mongodb JSON example data follows:

{
  "_id": "71ed6b0f-e08e-4bad-acf2-e02653aad1aa",
  "body": "This is my second post.",
  "type": "post",
  "slug": "prisma-is-type-safe-mongodb-client",
  "title": "Prisma is a type-safe MongoDB client"
}

and

{
  "_id": "7aa65cb4-e4b4-4a82-a26e-549d0bb5e3a7",
  "postId": "71ed6b0f-e08e-4bad-acf2-e02653aad1aa",
  "type": "comment",
  "comment": "Great post!"
}

And then, these two documents can be described as follows in the Prisma Schema:

model Post {
  id         String    @id @default(uuid()) @map("_id")
  type       String    @default("post")
  slug       String    @unique
  title      String
  body       String
  comments   Comment[]

  @@map("someCollection")
}

model Comment {
  id         String    @id @default(uuid()) @map("_id")
  type       String    @default("comment")
  post       Post      @relation(fields: [postId], references: [id])
  postId     String
  comment    String

  @@map("someCollection")
}

With this, most things work, except if you need to fetch a list of documents directly where the criteria overlaps between the variant types:

PrismaClientUnknownRequestError2 [PrismaClientUnknownRequestError]: 
Invalid `prisma.post.findMany()` invocation:


  Attempted to serialize scalar 'null' with incompatible type 'String'

Workaround

A work around is to add a filter to the findMany query to only return the documents matching the desired discriminator column:

await prisma.post.findMany({
  where: { type: "post" },
  include: {
    comments: true,
  },
})

Suggested solution

Add something like @discriminator to fields and then use it’s value whenever doing lookups for a model.

model Post {
  id         String    @id @default(uuid()) @map("_id")
  type       String    @default("post") @discriminator
  slug       String    @unique
  title      String
  body       String
  comments   Comment[]

  @@map("someCollection")
}

and so calling

await prisma.post.findMany({
  include: {
    comments: true,
  },
})

will work.

Additional context

More than one discriminator could also be very useful.

Perhaps the @discriminator could work like this: @discriminator("value") and automatically include @ignore on the field, maybe?

[matthewmueller]

Similar request to https://github.com/prisma/prisma/issues/2505

[SagnikPradhan]

Any updates on this issue?