How to Configure CASL Ability to Use __typename Instead of __caslSubjectType__ in TypeScript

[p0thi]

I'm working on setting up authorization in my TypeScript application using CASLand Prisma. In my setup, I use the detectSubjectType function to specify how to detect the subject type, and I expect each item to have the __typename field.

Here's the relevant part of my ability setup:

import {
  AbilityBuilder,
  AbilityOptionsOf,
  PureAbility,
  fieldPatternMatcher,
  AbilityTuple,
  Subjects,
} from "@casl/ability";
import { PrismaQuery, prismaQuery } from "@casl/prisma";

type Actions = "manage" | "read" | "update" | "create" | "delete";

type PrismaModels = {
  User: { id: string; name: string; /* other fields */ };
  Post: { id: string; title: string; /* other fields */ };
  // ... other models
};

type PrismaModelsWithTypename = {
  [K in keyof PrismaModels]: PrismaModels[K] & { __typename: K };
};

type AppSubjects = "all" | Subjects<PrismaModelsWithTypename>;

type AppAbility = PureAbility<AbilityTuple<Actions, AppSubjects>, PrismaQuery>;

export function createAppAbility(
  prisma: PrismaClient,
  authenticatedUser?: PrismaModels["User"] | null,
): AppAbility {
  const { can, cannot, build } = new AbilityBuilder<AppAbility>(PureAbility);

  const buildOptions: AbilityOptionsOf<AppAbility> = {
    fieldMatcher: fieldPatternMatcher,
    conditionsMatcher: prismaQuery,
    detectSubjectType: (item) => item.__typename,
  };

  // Define abilities
  can("read", "all");

  return build(buildOptions);
}

I also have an as function that adds the __typename field to a subject:

export function as<K extends keyof PrismaModels>(
  type: K,
  subject: PrismaModels[K],
): PrismaModelsWithTypename[K] {
  return { ...subject, __typename: type };
}

When I check permissions using the ability, I use the as function to ensure the subject has the __typename field:

const ability = createAppAbility(prisma, authenticatedUser);

const userToUpdate = { id: "1", name: "Alice" }; // Example user object

if (ability.can("update", as("User", userToUpdate))) {
  // Update the user
}

However, I'm running into a TypeScript error when using ability.can:

Argument of type '{ id: string; name: string; __typename: "User"; }' is not assignable to parameter of type 'AppSubjects'.
  Type '{ id: string; name: string; __typename: "User"; }' is not assignable to type 'ForcedSubject<"User">'.
    Property '__caslSubjectType__' is missing in type '{ id: string; name: string; __typename: "User"; }' but required in type 'ForcedSubject<"User">'.ts(2345)

It seems that CASL's ForcedSubject type expects the subject to have a __caslSubjectType__ property. But in my setup, I want to use __typename instead.

My Question:

• Is there a way to configure CASL so that it uses __typename for the subject type instead of __caslSubjectType__?
• How can I modify my setup so that the TypeScript types align correctly and I can use __typename as the subject type identifier?

Additional Information:

I understand that ForcedSubject is a type used internally by CASL to enforce the presence of the __caslSubjectType__ property for type checking. However, since I'm already using __typename in my detectSubjectType function, I'd like to avoid adding another property to my subjects if possible.

What I've Tried:

Attempted to adjust my types to include __caslSubjectType__, but this feels redundant since __typename is already serving that purpose in my setup.

Any help or guidance would be greatly appreciated!

---

Note: I'm using the following versions:

• TypeScript: 5.x
• CASL: 6.x
• Prisma: 5.x

Thank you!

[stalniy]

just don't use Subjects helper. You need to write custom type and it will work:

type Models = { User: User; Post: Post }
type ModelsWithTypename = { [K in keyof Models]: Models[K] & { __typename: K } } // <--- use this instead of Subjects