Reuse include in Prisma Query with TypeScript satisfies

The satisfies operator in TypeScript has been available since TypeScript 4.9, but I hadn't had the chance to use it in my production websites until a few days ago, when I finally did.

I have the following Prisma models:

model Participant {
  id                                       String            @id @default(uuid())
  createdAt                                DateTime          @default(now())
  participantGroup                         ParticipantGroup  @relation(fields: [participantGroupId], references: [id])
  participantGroupId                       String
}

model ParticipantGroup {
  id           String        @id @default(uuid())
  participants Participant[]
  course       Course[]

  @@map("participant_groups")
}

In one of my functions, I needed to:

1. Fetch a participant, including the participantGroup.
2. Perform some operations and then update the participant.
3. Pass the updated participant to a function, namely, the doSomethingWithParticipant function.

The doSomethingWithParticipant function requires the passed participant object to include the participantGroup, as follows:

// some-file.ts
const doSomethingFirst = async () => {
  let participant = await prismaClient.participant.findFirst({
    where: {
      id: 'some-uuid',
    },
    include: {
      participantGroup: {
        include: {
          course: true,
        },
      },
    },
  })

  // do some other operations
  participant = await prismaClient.participant.update({
    where: {
      id: 'some-uuid',
    },
    data: {
      // update participant's data
    },
    include: {
      participantGroup: {
        include: {
          course: true,
        },
      },
    },
  })

  await doSomethingWithParticipant(participant)
}

// some-file2.ts
type ParticipantWithGroup = Prisma.ParticipantGetPayload<{
  include: {
    participantGroup: true
  }
}>

const doSomethingWithParticipant = async (participant: ParticipantWithGroup) => {
  // access participant.participantGroup here
}

As you can see, I had to repeat the include property in both the findFirst and update methods. If I didn't, TypeScript would raise an issue near the doSomethingWithParticipant function. Let's try to refactor the code by assigning the include property to a variable.

const relationToInclude = {
  participantGroup: {
    include: {
      course: true,
    },
  },
}
const doSomethingFirst = async () => {
  let participant = await prismaClient.participant.findFirst({
    where: {
      id: 'some-uuid',
    },
    include: relationToInclude,
  })

  // do some other operations
  participant = await prismaClient.participant.update({
    where: {
      id: 'some-uuid',
    },
    data: {
      // update participant's data
    },
    include: relationToInclude,
  })

  await doSomethingWithParticipant(participant)
}

Great, now I no longer have repeating code. Additionally, TypeScript reminds me if I make a typo:

Image

However, I've lost the autocomplete feature, and I can assign unknown properties to the relationToInclude object:

const relationToInclude = {
  participantGroup: {
    include: {
      course: true,
    },
  },
  randomStuff: 'true', // this is not correct
}

So, my first thought was to type the relationToInclude variable directly as Prisma.ParticipantInclude:

const relationToInclude: Prisma.ParticipantInclude = {
  participantGroup: {
    include: {
      course: true,
    },
  },
  randomStuff: 'what', // great, TypeScript showed error here!
}

It worked, and TypeScript threw an error for randomStuff because of randomStuff. However, TypeScript also generated another error at the line where the doSomethingWithParticipant function is called:

Image

This issue arose because the exact type of Prisma.ParticipantInclude makes participantGroup optional, which means it can be undefined, even though the object has participantGroup at runtime. Since TypeScript perceives participantGroup in relationToInclude as potentially undefined, the findFirst and update queries infer that the returned participant might not include participantGroup. Meanwhile, the doSomethingWithParticipant function expects participantGroup to be defined.

This is where the satisfies operator becomes useful. Instead of directly typing relationToInclude as Prisma.ParticipantInclude, we can use the satisfies operator to assert that this object meets the Prisma.ParticipantInclude structure.

const relationToInclude = {
  participantGroup: {
    include: {
      course: true,
    },
  },
} satisfies Prisma.ParticipantInclude

This approach allows TypeScript to perform type checking on relationToInclude, while also enabling it to infer the exact type of relationToInclude in the findFirst and update queries. Consequently, TypeScript will recognize that the updated participant includes participantGroup.

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