One-to-many relations - Prisma

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

prisma

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:

prisma

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

Multi-field relations (relational databases only)

prisma

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):

prisma

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):

prisma

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
}