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Self-relations

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.

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

One-to-one self-relations​

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

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

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. 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:

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

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

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

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

const x = await prisma.user.create({
  data: {
    name: "Bob McBob",
      successor: {
      connect: {
        id: 2,
      },
    },
      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:

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, 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:

{ "_id": { "$oid": "60d97df70080618f000e3ca9" }, "name": "Elsa the Elder" }
{
  "_id": { "$oid": "60d97df70080618f000e3caa" },
  "name": "Elsa",
  "successorId": { "$oid": "60d97df70080618f000e3ca9" }
}

One-to-many self relations​

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

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

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.

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:

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, 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:

{
  "_id": { "$oid": "60d9b9e600fe3d470079d6f9" },
  "name": "Ms. Roberts"
}
{
  "_id": { "$oid": "60d9b9e600fe3d470079d6fa" },
  "name": "Student 8",
  "teacherId": { "$oid": "60d9b9e600fe3d470079d6f9" }
}
{
  "_id": { "$oid": "60d9b9e600fe3d470079d6fb" },
  "name": "Student 9",
  "teacherId": { "$oid": "60d9b9e600fe3d470079d6f9" }
}

Many-to-many self relations​

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

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

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. This means Prisma ORM maintains a relation table for it in the underlying database.

If you need the relation to hold other fields, you can create an explicit many-to-many self relation as well. The explicit version of the self relation shown previously is as follows:

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:

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, 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:

{
  "_id": { "$oid": "60d9866f00a3e930009a6cdd" },
  "name": "Bob",
  "followedByIDs": [
    { "$oid": "60d9866f00a3e930009a6cde" },
    { "$oid": "60d9867000a3e930009a6cdf" }
  ],
  "followingIDs": [
    { "$oid": "60d9867000a3e930009a6ce0" },
    { "$oid": "60d9867000a3e930009a6ce1" }
  ]
}
{
  "_id": { "$oid": "60d9866f00a3e930009a6cde" },
  "name": "Follower1",
  "followingIDs": [{ "$oid": "60d9866f00a3e930009a6cdd" }]
}
{
  "_id": { "$oid": "60d9867000a3e930009a6cdf" },
  "name": "Follower2",
  "followingIDs": [{ "$oid": "60d9866f00a3e930009a6cdd" }]
}
{
  "_id": { "$oid": "60d9867000a3e930009a6ce0" },
  "name": "CoolPerson1",
  "followedByIDs": [{ "$oid": "60d9866f00a3e930009a6cdd" }]
}
{
  "_id": { "$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:

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")
}