One-to-one relations

Source URL: https://docs.prisma.io/docs/orm/prisma-schema/data-model/relations/one-to-one-relations

One-to-one relations

How to define and work with one-to-one relations in Prisma.

One-to-one (1-1) relations connect at most one record on each side. In this example, User and Profile have a 1-1 relation:

    model User {
      id      Int      @id @default(autoincrement())
      profile Profile?
    }

    model Profile {
      id     Int  @id @default(autoincrement())
      user   User @relation(fields: [userId], references: [id])
      userId Int  @unique // Foreign key with unique constraint
    }

This expresses:

• A user can have zero or one profile
• A profile must always be connected to exactly one user

You can also reference a non-ID field with @unique:

    model Profile {
      id        Int    @id @default(autoincrement())
      user      User   @relation(fields: [userEmail], references: [email])
      userEmail String @unique
    }

Multi-field relations (relational databases only)

    model User {
      firstName String
      lastName  String
      profile   Profile?
      @@id([firstName, lastName])
    }

    model Profile {
      id            Int    @id @default(autoincrement())
      user          User   @relation(fields: [userFirstName, userLastName], references: [firstName, lastName])
      userFirstName String
      userLastName  String
      @@unique([userFirstName, userLastName])
    }

1-1 in the database

In SQL, a 1-1 relation requires a UNIQUE constraint on the foreign key. Without this, it becomes a 1-n relation.

For MongoDB, documents reference each other by ID:

    // User
    { "_id": { "$oid": "60d58e130011041800d209e1" }, "name": "Bob" }
    // Profile
    { "_id": "...", "bio": "I like drawing.", "userId": { "$oid": "60d58e130011041800d209e1" } }

Required and optional 1-1 relation fields

The side without a relation scalar must be optional:

    model User {
      id      Int      @id @default(autoincrement())
      profile Profile? // No relation scalar - must be optional
    }

The side with a relation scalar can be required or optional:

Mandatory 1-1 (cannot create User without Profile):

    model User {
      id        Int     @id @default(autoincrement())
      profile   Profile @relation(fields: [profileId], references: [id])
      profileId Int     @unique
    }

Optional 1-1 (can create User without Profile):

    model User {
      id        Int      @id @default(autoincrement())
      profile   Profile? @relation(fields: [profileId], references: [id])
      profileId Int?     @unique
    }

Choosing which side stores the foreign key

In 1-1 relations, you can choose which side holds the @relation attribute and foreign key. Both approaches are valid:

Option 1: Foreign key on Profile

    model User {
      id      Int      @id @default(autoincrement())
      profile Profile?
    }

    model Profile {
      id     Int  @id @default(autoincrement())
      user   User @relation(fields: [userId], references: [id])
      userId Int  @unique
    }

Option 2: Foreign key on User

    model User {
      id        Int      @id @default(autoincrement())
      profile   Profile? @relation(fields: [profileId], references: [id])
      profileId Int?     @unique
    }

    model Profile {
      id   Int   @id @default(autoincrement())
      user User?
    }