Prisma ORM v8 removes required empty fields from field-less Mongo variant during contract emit

[Shgit29]

Package and version

@internal/mongo-contract-psl — prisma/prisma main @ 10c57bc

What happened?

This is a re-file of prisma/prisma-next#1070 after Prisma Next development moved to prisma/prisma.

The issue still reproduces on the latest main.

For a Mongo polymorphic variant with no fields of its own, the PSL interpreter correctly produces fields: {}. Generic contract canonicalization then removes that empty object.

The resulting contract is invalid because fields is required by the Mongo contract schema.

Observed result:

BEFORE_NOTE={"fields":{},"relations":{},"storage":{"collection":"posts"},"base":{"namespace":"__unbound__","model":"Post"}}
BEFORE_HAS_FIELDS=true

AFTER_NOTE={"base":{"model":"Post","namespace":"__unbound__"},"relations":{},"storage":{"collection":"posts"}}
AFTER_HAS_FIELDS=false

CONTRACT_VALID=false
VALIDATION_ERROR=domain.namespaces.__unbound__.models.Note.fields must be an object (was missing)

What did you expect to happen?

Canonicalization should preserve the required empty fields object for a field-less variant.

The canonicalized contract should therefore retain:

{
  "fields": {}
}

and pass MongoContractSchema validation.

Minimal reproduction

Using the current Mongo PSL interpreter, canonicalizer, and contract schema with:

model Post {
  id    ObjectId @id @map("_id")
  title String
  kind  String

  @@discriminator(kind)
  @@map("posts")
}

model Note {
  @@base(Post, "note")
}

1. Interpret the schema through the Mongo PSL interpreter.
2. Verify that Note.fields is {}.
3. Canonicalize the resulting contract.
4. Inspect Note.fields again.
5. Validate the canonicalized contract with MongoContractSchema.

Current result:

Before canonicalization: Note.fields exists and equals {}
After canonicalization: Note.fields is missing
MongoContractSchema validation: fails

Environment

• Prisma repository: prisma/prisma
• Commit: 10c57bcca34f1bd10ddd01571f82fa9ce4280841
• Branch: main
• Node: 24.19.0
• OS: Fedora Linux
• Package manager: pnpm
• Target database family: MongoDB

Additional context

Original report: prisma/prisma-next#1070

The issue was closed because the prisma/prisma-next repository is being archived, not because the bug was fixed.

I re-tested this against the latest prisma/prisma main, where it still reproduces.

The current canonicalizer has preservation handling for required empty model relations and storage objects, but no equivalent handling for fields.

A PR with a fix and regression coverage is already open: #30003