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.

Deriving a Full Name from a First and Last Name

Prisma Client does not yet natively support computed fields, but with a bit of TypeScript magic, you can define a function that accepts a generic 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 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!