Relations

A relation is a connection between two models in the Prisma schema. For example, there is a one-to-many relation between User and Post because one user can have many blog posts.

The following Prisma schema defines a one-to-many relation between the User and Post models. The fields involved in defining the relation are highlighted:

model User {
  id    Int    @id @default(autoincrement())
  posts Post[]
}

model Post {
  id       Int  @id @default(autoincrement())
  author   User @relation(fields: [authorId], references: [id])
  authorId Int // relation scalar field  (used in the `@relation` attribute above)
}

At a Prisma level, the User / Post relation is made up of:

• Two relation fields: author and posts. Relation fields define connections between models at the Prisma level and do not exist in the database. These fields are used to generate Prisma Client.
• The scalar authorId field, which is referenced by the @relation attribute. This field does exist in the database - it is the foreign key that connects Post and User.

At a Prisma level, a connection between two models is always represented by a relation field on each side of the relation.

Relations in the database

author     User        @relation(fields: [authorId], references: [id])

{ "_id": { "$oid": "60d5922d00581b8f0062e3a8" }, "name": "Ella" }

[
  {
    "_id": { "$oid": "60d5922e00581b8f0062e3a9" },
    "title": "How to make sushi",
    "authorId": { "$oid": "60d5922d00581b8f0062e3a8" }
  },
  {
    "_id": { "$oid": "60d5922e00581b8f0062e3aa" },
    "title": "How to re-install Windows",
    "authorId": { "$oid": "60d5922d00581b8f0062e3a8" }
  }
]

model User {
  id    String @id @default(auto()) @map("_id") @db.ObjectId
  posts Post[]
}

model Post {
  id       String @id @default(auto()) @map("_id") @db.ObjectId
  author   User   @relation(fields: [authorId], references: [id])
  authorId String @db.ObjectId // relation scalar field  (used in the `@relation` attribute above)
}

Relations in Prisma Client

Prisma Client is generated from the Prisma schema. The following examples demonstrate how relations manifest when you use Prisma Client to get, create, and update records.

The following query creates a User record and two connected Post records:

const userAndPosts = await prisma.user.create({
  data: {
    posts: {
      create: [
        { title: 'Prisma Day 2020' }, // Populates authorId with user's id
        { title: 'How to write a Prisma schema' }, // Populates authorId with user's id
      ],
    },
  },
})

In the underlying database, this query:

1. Creates a User with an auto-generated id (for example, 20)
2. Creates two new Post records and sets the authorId of both records to 20

The following query retrieves a User by id and includes any related Post records:

const getAuthor = await prisma.user.findUnique({
  where: {
    id: "20",
  },
  include: {
    posts: true, // All posts where authorId == 20
  },
});

In the underlying database, this query:

1. Retrieves the User record with an id of 20
2. Retrieves all Post records with an authorId of 20

The following query associates an existing Post record with an existing User record:

const updateAuthor = await prisma.user.update({
  where: {
    id: 20,
  },
  data: {
    posts: {
      connect: {
        id: 4,
      },
    },
  },
})

In the underlying database, this query uses a nested connect query to link the post with an id of 4 to the user with an id of 20. The query does this with the following steps:

• The query first looks for the user with an id of 20.
• The query then sets the authorID foreign key to 20. This links the post with an id of 4 to the user with an id of 20.

In this query, the current value of authorID does not matter. The query changes authorID to 20, no matter its current value.

Types of relations

There are three different types (or cardinalities) of relations in Prisma:

• One-to-one (also called 1-1 relations)
• One-to-many (also called 1-n relations)
• Many-to-many (also called m-n relations)

The following Prisma schema includes every type of relation:

• one-to-one: User ↔ Profile
• one-to-many: User ↔ Post
• many-to-many: Post ↔ Category

model User {
  id      Int      @id @default(autoincrement())
  posts   Post[]
  profile Profile?
}

model Profile {
  id     Int  @id @default(autoincrement())
  user   User @relation(fields: [userId], references: [id])
  userId Int  @unique // relation scalar field (used in the `@relation` attribute above)
}

model Post {
  id         Int        @id @default(autoincrement())
  author     User       @relation(fields: [authorId], references: [id])
  authorId   Int // relation scalar field  (used in the `@relation` attribute above)
  categories Category[]
}

model Category {
  id    Int    @id @default(autoincrement())
  posts Post[]
}

Notice that the syntax is slightly different between relational databases and MongoDB - particularly for many-to-many relations.

For relational databases, the following entity relationship diagram represents the database that corresponds to the sample Prisma schema:

For MongoDB, Prisma uses a normalized data model design, which means that documents reference each other by ID in a similar way to relational databases. See the MongoDB section for more details.

Relation fields

model User {
  id    Int    @id @default(autoincrement())
  email String @unique
  role  Role   @default(USER)
  posts Post[]
}

model Post {
  id       Int    @id @default(autoincrement())
  title    String
  author   User   @relation(fields: [authorId], references: [id])
  authorId Int // relation scalar field (used in the `@relation` attribute above)
}

These models have the following fields:

Both posts and author are relation fields because their types are not scalar types but other models.

Also note that the annotated relation field author needs to link the relation scalar field authorId on the Post model inside the @relation attribute. The relation scalar represents the foreign key in the underlying database.

The other relation field called posts is defined purely on a Prisma-level, it doesn't manifest in the database.

author     User    @relation(fields: [authorId], references: [id])
authorId   Int

The @relation attribute

The @relation attribute can only be applied to the relation fields, not to scalar fields.

The @relation attribute is required when:

• you define a one-to-one or one-to-many relation, it is required on one side of the relation (with the corresponding relation scalar field)
• you need to disambiguate a relation (that's e.g. the case when you have two relations between the same models)
• you define a self-relation
• you define a many-to-many relation for MongoDB
• you need to control how the relation table is represented in the underlying database (e.g. use a specific name for a relation table)

Note: Implicit many-to-many relations in relational databases do not require the @relation attribute.

Disambiguating relations

When you define two relations between the same two models, you need to add the name argument in the @relation attribute to disambiguate them. As an example for why that's needed, consider the following models:

model User {
  id           Int     @id @default(autoincrement())
  name         String?
  writtenPosts Post[]
  pinnedPost   Post?
}

model Post {
  id         Int     @id @default(autoincrement())
  title      String?
  author     User    @relation(fields: [authorId], references: [id])
  authorId   Int
  pinnedBy   User?   @relation(fields: [pinnedById], references: [id])
  pinnedById Int?
}

In that case, the relations are ambiguous, there are four different ways to interpret them:

• User.writtenPosts ↔ Post.author + Post.authorId
• User.writtenPosts ↔ Post.pinnedBy + Post.pinnedById
• User.pinnedPost ↔ Post.author + Post.authorId
• User.pinnedPost ↔ Post.pinnedBy + Post.pinnedById

To disambiguate these relations, you need to annotate the relation fields with the @relation attribute and provide the name argument. You can set any name (except for the empty string ""), but it must be the same on both sides of the relation:

model User {
  id           Int     @id @default(autoincrement())
  name         String?
  writtenPosts Post[]  @relation("WrittenPosts")
  pinnedPost   Post?   @relation("PinnedPost")
}

model Post {
  id         Int     @id @default(autoincrement())
  title      String?
  author     User    @relation("WrittenPosts", fields: [authorId], references: [id])
  authorId   Int
  pinnedBy   User?   @relation("PinnedPost", fields: [pinnedById], references: [id])
  pinnedById Int?    @unique
}