# One-to-one relations (Prisma ORM v7) (/docs/orm/v7/prisma-schema/data-model/relations/one-to-one-relations)

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

Location: ORM > v7 > Prisma Schema > Data Model > Relations > One-to-one relations

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:

```prisma
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`:

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

## Multi-field relations (relational databases only) [#multi-field-relations-relational-databases-only]

```prisma
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 [#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:

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

## Required and optional 1-1 relation fields [#required-and-optional-1-1-relation-fields]

The side *without* a relation scalar must be optional:

```prisma
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):

```prisma
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):

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

## Choosing which side stores the foreign key [#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`

```prisma
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`

```prisma
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?
}
```

## Related pages

- [`Many-to-many relations`](https://www.prisma.io/docs/orm/v7/prisma-schema/data-model/relations/many-to-many-relations): How to define and work with many-to-many relations in Prisma.
- [`One-to-many relations`](https://www.prisma.io/docs/orm/v7/prisma-schema/data-model/relations/one-to-many-relations): How to define and work with one-to-many relations in Prisma.
- [`Referential actions`](https://www.prisma.io/docs/orm/v7/prisma-schema/data-model/relations/referential-actions): Referential actions let you define the update and delete behavior of related models on the database level
- [`Relation mode`](https://www.prisma.io/docs/orm/v7/prisma-schema/data-model/relations/relation-mode): Manage relations between records with relation modes in Prisma
- [`Self-relations`](https://www.prisma.io/docs/orm/v7/prisma-schema/data-model/relations/self-relations): How to define and work with self-relations in Prisma.