A relation field can also reference its own model, in this case the relation is called a _self-relation_. Self-relations can be of any cardinality, 1-1, 1-n and m-n.

<details>
<summary>Questions answered in this page</summary>

- How do I model self-relations?
- Do self-relations always require @relation?
- How do 1-1 vs 1-n self-relations differ?

</details>

Note that self-relations always require the `@relation` attribute.

## One-to-one self-relations

The following example models a one-to-one self-relation:

<TabbedContent code>
<TabItem value="Relational databases">

```prisma
model User {
  id          Int     @id @default(autoincrement())
  name        String?
  successorId Int?    @unique
  successor   User?   @relation("BlogOwnerHistory", fields: [successorId], references: [id])
  predecessor User?   @relation("BlogOwnerHistory")
}
```

</TabItem>
<TabItem value="MongoDB">

```prisma
model User {
  id          String  @id @default(auto()) @map("_id") @db.ObjectId
  name        String?
  successorId String? @unique @db.ObjectId
  successor   User?   @relation("BlogOwnerHistory", fields: [successorId], references: [id])
  predecessor User?   @relation("BlogOwnerHistory")
}
```

</TabItem>
</TabbedContent>

This relation expresses the following:

- "a user can have one or zero predecessors" (for example, Sarah is Mary's predecessor as blog owner)
- "a user can have one or zero successors" (for example, Mary is Sarah's successor as blog owner)

> **Note**: One-to-one self-relations cannot be made required on both sides. One or both sides must be optional, otherwise it becomes impossible to create the first `User` record.

To create a one-to-one self-relation:

- Both sides of the relation must define a `@relation` attribute that share the same name - in this case, **BlogOwnerHistory**.
- One relation field must be a [fully annotated](/orm/prisma-schema/data-model/relations#relation-fields). In this example, the `successor` field defines both the `field` and `references` arguments.
- One relation field must be backed by a foreign key. The `successor` field is backed by the `successorId` foreign key, which references a value in the `id` field. The `successorId` scalar relation field also requires a `@unique` attribute to guarantee a one-to-one relation.

> **Note**: One-to-one self relations require two sides even if both sides are equal in the relationship. For example, to model a 'best friends' relation, you would need to create two relation fields: `bestfriend1` and a `bestfriend2`.

Either side of the relation can be backed by a foreign key. In the previous example, repeated below, `successor` is backed by `successorId`:

<TabbedContent code>
<TabItem value="Relational databases">

```prisma highlight=4;normal
model User {
  id          Int     @id @default(autoincrement())
  name        String?
  //highlight-next-line
  successorId Int?    @unique
  successor   User?   @relation("BlogOwnerHistory", fields: [successorId], references: [id])
  predecessor User?   @relation("BlogOwnerHistory")
}
```

</TabItem>
<TabItem value="MongoDB">

```prisma
model User {
  id          String  @id @default(auto()) @map("_id") @db.ObjectId
  name        String?
  //highlight-next-line
  successorId String? @unique @db.ObjectId
  successor   User?   @relation("BlogOwnerHistory", fields: [successorId], references: [id])
  predecessor User?   @relation("BlogOwnerHistory")
}
```

</TabItem>
</TabbedContent>

Alternatively, you could rewrite this so that `predecessor` is backed by `predecessorId`:

<TabbedContent code>
<TabItem value="Relational databases">

```prisma
model User {
  id            Int     @id @default(autoincrement())
  name          String?
  successor     User?   @relation("BlogOwnerHistory")
  //highlight-start
  predecessorId Int?    @unique
  predecessor   User?   @relation("BlogOwnerHistory", fields: [predecessorId], references: [id])
  //highlight-end
}
```

</TabItem>
<TabItem value="MongoDB">

```prisma
model User {
  id            String  @id @default(auto()) @map("_id") @db.ObjectId
  name          String?
  successor     User?   @relation("BlogOwnerHistory")
  //highlight-start
  predecessorId String? @unique @db.ObjectId
  predecessor   User?   @relation("BlogOwnerHistory", fields: [predecessorId], references: [id])
  //highlight-end
}
```

</TabItem>
</TabbedContent>

No matter which side is backed by a foreign key, Prisma Client surfaces both the `predecessor` and `successor` fields:

```ts showLineNumbers
const x = await prisma.user.create({
  data: {
    name: "Bob McBob",
      //highlight-next-line
      successor: {
      connect: {
        id: 2,
      },
    },
      //highlight-next-line
      predecessor: {
      connect: {
        id: 4,
      },
    },
  },
});
```

### One-to-one self relations in the database

### Relational databases

In **relational databases only**, a one-to-one self-relation is represented by the following SQL:

```sql
CREATE TABLE "User" (
    id SERIAL PRIMARY KEY,
    "name" TEXT,
    "successorId" INTEGER
);

ALTER TABLE "User" ADD CONSTRAINT fk_successor_user FOREIGN KEY ("successorId") REFERENCES "User" (id);

ALTER TABLE "User" ADD CONSTRAINT successor_unique UNIQUE ("successorId");
```

### MongoDB

For MongoDB, Prisma ORM currently uses a [normalized data model design](https://www.mongodb.com/docs/manual/data-modeling/), which means that documents reference each other by ID in a similar way to relational databases.

The following MongoDB documents represent a one-to-one self-relation between two users:

```json
{ "_id": { "$oid": "60d97df70080618f000e3ca9" }, "name": "Elsa the Elder" }
```

```json
{
  "_id": { "$oid": "60d97df70080618f000e3caa" },
  "name": "Elsa",
  "successorId": { "$oid": "60d97df70080618f000e3ca9" }
}
```

## One-to-many self relations

A one-to-many self-relation looks as follows:

<TabbedContent code>
<TabItem value="Relational databases">

```prisma
model User {
  id        Int     @id @default(autoincrement())
  name      String?
  teacherId Int?
  teacher   User?   @relation("TeacherStudents", fields: [teacherId], references: [id])
  students  User[]  @relation("TeacherStudents")
}
```

</TabItem>
<TabItem value="MongoDB">

```prisma
model User {
  id        String  @id @default(auto()) @map("_id") @db.ObjectId
  name      String?
  teacherId String? @db.ObjectId
  teacher   User?   @relation("TeacherStudents", fields: [teacherId], references: [id])
  students  User[]  @relation("TeacherStudents")
}
```

</TabItem>
</TabbedContent>

This relation expresses the following:

- "a user has zero or one _teachers_ "
- "a user can have zero or more _students_"

Note that you can also require each user to have a teacher by making the `teacher` field [required](/orm/prisma-schema/data-model/models#optional-and-mandatory-fields).

### One-to-many self-relations in the database

### Relational databases

In relational databases, a one-to-many self-relation is represented by the following SQL:

```sql
CREATE TABLE "User" (
    id SERIAL PRIMARY KEY,
    "name" TEXT,
    "teacherId" INTEGER
);

ALTER TABLE "User" ADD CONSTRAINT fk_teacherid_user FOREIGN KEY ("teacherId") REFERENCES "User" (id);
```

Notice the lack of `UNIQUE` constraint on `teacherId` - multiple students can have the same teacher.

### MongoDB

For MongoDB, Prisma ORM currently uses a [normalized data model design](https://www.mongodb.com/docs/manual/data-modeling/), which means that documents reference each other by ID in a similar way to relational databases.

The following MongoDB documents represent a one-to-many self-relation between three users - one teacher and two students with the same `teacherId`:

```json
{
  "_id": { "$oid": "60d9b9e600fe3d470079d6f9" },
  "name": "Ms. Roberts"
}
```

```json
{
  "_id": { "$oid": "60d9b9e600fe3d470079d6fa" },
  "name": "Student 8",
  "teacherId": { "$oid": "60d9b9e600fe3d470079d6f9" }
}
```

```json
{
  "_id": { "$oid": "60d9b9e600fe3d470079d6fb" },
  "name": "Student 9",
  "teacherId": { "$oid": "60d9b9e600fe3d470079d6f9" }
}
```

## Many-to-many self relations

A many-to-many self-relation looks as follows:

<TabbedContent code>
<TabItem value="Relational databases">

```prisma
model User {
  id         Int     @id @default(autoincrement())
  name       String?
  followedBy User[]  @relation("UserFollows")
  following  User[]  @relation("UserFollows")
}
```

</TabItem>
<TabItem value="MongoDB">

```prisma
model User {
  id            String   @id @default(auto()) @map("_id") @db.ObjectId
  name          String?
  followedBy    User[]   @relation("UserFollows", fields: [followedByIDs], references: [id])
  followedByIDs String[] @db.ObjectId
  following     User[]   @relation("UserFollows", fields: [followingIDs], references: [id])
  followingIDs  String[] @db.ObjectId
}
```

</TabItem>
</TabbedContent>

This relation expresses the following:

- "a user can be followed by zero or more users"
- "a user can follow zero or more users"

Note that for relational databases, this many-to-many-relation is [implicit](/orm/prisma-schema/data-model/relations/many-to-many-relations#implicit-many-to-many-relations). This means Prisma ORM maintains a [relation table](/orm/prisma-schema/data-model/relations/many-to-many-relations#relation-tables) for it in the underlying database.

If you need the relation to hold other fields, you can create an [explicit](/orm/prisma-schema/data-model/relations/many-to-many-relations#explicit-many-to-many-relations) many-to-many self relation as well. The explicit version of the self relation shown previously is as follows:

```prisma
model User {
  id         Int       @id @default(autoincrement())
  name       String?
  followedBy Follows[] @relation("followedBy")
  following  Follows[] @relation("following")
}

model Follows {
  followedBy   User @relation("followedBy", fields: [followedById], references: [id])
  followedById Int
  following    User @relation("following", fields: [followingId], references: [id])
  followingId  Int

  @@id([followingId, followedById])
}
```

### Many-to-many self-relations in the database

### Relational databases

In relational databases, a many-to-many self-relation (implicit) is represented by the following SQL:

```sql
CREATE TABLE "User" (
    id integer DEFAULT nextval('"User_id_seq"'::regclass) PRIMARY KEY,
    name text
);
CREATE TABLE "_UserFollows" (
    "A" integer NOT NULL REFERENCES "User"(id) ON DELETE CASCADE ON UPDATE CASCADE,
    "B" integer NOT NULL REFERENCES "User"(id) ON DELETE CASCADE ON UPDATE CASCADE
);
```

### MongoDB

For MongoDB, Prisma ORM currently uses a [normalized data model design](https://www.mongodb.com/docs/manual/data-modeling/), which means that documents reference each other by ID in a similar way to relational databases.

The following MongoDB documents represent a many-to-many self-relation between five users - two users that follow `"Bob"`, and two users that follow him:

```json
{
  "_id": { "$oid": "60d9866f00a3e930009a6cdd" },
  "name": "Bob",
  "followedByIDs": [
    { "$oid": "60d9866f00a3e930009a6cde" },
    { "$oid": "60d9867000a3e930009a6cdf" }
  ],
  "followingIDs": [
    { "$oid": "60d9867000a3e930009a6ce0" },
    { "$oid": "60d9867000a3e930009a6ce1" }
  ]
}
```

```json
{
  "_id": { "$oid": "60d9866f00a3e930009a6cde" },
  "name": "Follower1",
  "followingIDs": [{ "$oid": "60d9866f00a3e930009a6cdd" }]
}
```

```json
{
  "_id": { "$oid": "60d9867000a3e930009a6cdf" },
  "name": "Follower2",
  "followingIDs": [{ "$oid": "60d9866f00a3e930009a6cdd" }]
}
```

```json
{
  "_id": { "$oid": "60d9867000a3e930009a6ce0" },
  "name": "CoolPerson1",
  "followedByIDs": [{ "$oid": "60d9866f00a3e930009a6cdd" }]
}
```

```json
{
  "_id": { "$oid": "60d9867000a3e930009a6ce1" },
  "name": "CoolPerson2",
  "followedByIDs": [{ "$oid": "60d9866f00a3e930009a6cdd" }]
}
```

## Defining multiple self-relations on the same model

You can also define multiple self-relations on the same model at once. Taking all relations from the previous sections as example, you could define a `User` model as follows:

<TabbedContent code>
<TabItem value="Relational databases">

```prisma
model User {
  id         Int     @id @default(autoincrement())
  name       String?
  teacherId  Int?
  teacher    User?   @relation("TeacherStudents", fields: [teacherId], references: [id])
  students   User[]  @relation("TeacherStudents")
  followedBy User[]  @relation("UserFollows")
  following  User[]  @relation("UserFollows")
}
```

</TabItem>
<TabItem value="MongoDB">

```prisma
model User {
  id            String   @id @default(auto()) @map("_id") @db.ObjectId
  name          String?
  teacherId     String?  @db.ObjectId
  teacher       User?    @relation("TeacherStudents", fields: [teacherId], references: [id])
  students      User[]   @relation("TeacherStudents")
  followedBy    User[]   @relation("UserFollows", fields: [followedByIDs])
  followedByIDs String[] @db.ObjectId
  following     User[]   @relation("UserFollows", fields: [followingIDs])
  followingIDs  String[] @db.ObjectId
}
```

</TabItem>
</TabbedContent>

---

# Special rules for referential actions in SQL Server and MongoDB