Establishing Model Relations in Prisma

Relational databases structure data using foreign key references. Prisma models support three types of database relationships: One-to-One, One-to-Many, and Many-to-Many.

1. One-to-Many Relationships (User and Posts)

A one-to-many relationship is the most common association. For example, a User can author multiple posts, but each Post belongs to a single author.

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

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

• posts Post[] declares the list connection.
• @relation(fields: [authorId], references: [id]) binds the local authorId foreign key to the primary key id of the User model.

2. One-to-One Relationships (User and Profile)

In a one-to-one relationship, each record in one table maps to a single record in another. For example, a User has a single Profile.

To enforce this relationship, make the foreign key field @unique in your schema:

model User {
  id      Int      @id @default(autoincrement())
  profile Profile?
}

model Profile {
  id     Int    @id @default(autoincrement())
  bio    String
  userId Int    @unique
  user   User   @relation(fields: [userId], references: [id])
}

3. Many-to-Many Relationships (Post and Tags)

In many-to-many relationships, multiple records in one table map to multiple records in another. For example, a Post can have multiple Tags, and a Tag can belong to multiple Posts.

Prisma supports implicit many-to-many relationships, which automatically manage the join table under the hood:

model Post {
  id   Int   @id @default(autoincrement())
  tags Tag[]
}

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

If you need to store extra fields (such as a timestamp) on the relationship itself, define an explicit join model instead:

model Post {
  id   Int       @id @default(autoincrement())
  tags PostToTag[]
}

model Tag {
  id    Int       @id @default(autoincrement())
  posts PostToTag[]
}

model PostToTag {
  postId Int
  tagId  Int
  post   Post @relation(fields: [postId], references: [id])
  tag    Tag  @relation(fields: [tagId], references: [id])

  @@id([postId, tagId])
}