One-to-many relations

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

One-to-many relations

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

One-to-many (1-n) relations connect one record on one side to zero or more records on the other side:

    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
    }

This expresses:

• A user can have zero or more posts
• A post must always have an author

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

    model Post {
      id          Int    @id @default(autoincrement())
      authorEmail String
      author      User   @relation(fields: [authorEmail], references: [email])
    }

Multi-field relations (relational databases only)

    model User {
      firstName String
      lastName  String
      post      Post[]
      @@id([firstName, lastName])
    }

    model Post {
      id              Int    @id @default(autoincrement())
      author          User   @relation(fields: [authorFirstName, authorLastName], references: [firstName, lastName])
      authorFirstName String
      authorLastName  String
    }

1-n in the database

The difference between 1-1 and 1-n is that in a 1-1 relation the foreign key must have a UNIQUE constraint. Without UNIQUE, multiple records can point to the same parent, making it 1-n.

Required vs optional relation fields

The annotated relation field and relation scalar can be either optional or mandatory. The list side is always mandatory.

Optional 1-n (can create Post without User):

    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?
    }

Mandatory 1-n (must assign User when creating Post):

    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
    }