Computed fields

Computed fields allow you to derive a new field based on existing data. A common example is when you compute a full name from a first and last name. In your database, you may only store the first and last name, but you can define a function that computes a full name by combining the first and last name. This field is read-only and stored in your application's memory, not in your database.

Using a Prisma Client extension

Prisma Client extensions are currently in Preview.

The following example illustrates how to create a Prisma Client extension that adds a fullName computed field at runtime to the User model in a Prisma schema.

import { PrismaClient } from '@prisma/client'

const prisma = new PrismaClient().$extends({
  result: {
    user: {
      fullName: {
        needs: { firstName: true, lastName: true },
        compute(user) {
          return `${user.firstName} ${user.lastName}`
        },
      },
    },
  },
})

async function main() {
  /**
   * Example query containing the `fullName` computed field in the response
   */
  const user = await prisma.user.findFirst()
}

main()

The computed fields are type-safe and can return anything from a concatenated value to complex objects or functions that can act as an instance method for your models.

Using a computation function

Prisma Client does not yet natively support computed fields, but, you can define a function that accepts a generic type as an input then extend that generic to ensure it conforms to a specific structure. Finally, you can return that generic with additional computed fields. Let's see how that might look:

// Define a type that needs a first and last name
type FirstLastName = {
  firstName: string
  lastName: string
}

// Extend the T generic with the fullName attribute
type WithFullName<T> = T & {
  fullName: string
}

// Take objects that satisfy FirstLastName and computes a full name
function computeFullName<User extends FirstLastName>(
  user: User
): WithFullName<User> {
  return {
    ...user,
    fullName: user.firstName + ' ' + user.lastName,
  }
}

async function main() {
  const user = await prisma.user.findUnique({ where: 1 })
  const userWithFullName = computeFullName(user)
}

In the TypeScript example above, a User generic has been defined that extends the FirstLastName type. This means that whatever you pass into computeFullName must contain firstName and lastName keys.

A WithFullName<User> return type has also been defined, which takes whatever User is and tacks on a fullName string attribute.

With this function, any object that contains firstName and lastName keys can compute a fullName. Pretty neat, right?

Going further

• Learn how you can use Prisma Client extensions to add a computed field to your schema — example.
• Learn how you can move the computeFullName function into a custom model.
• There's an outstanding feature request to add native support to Prisma Client. If you'd like to see that happen, make sure to upvote that issue and share your use case!