One-to-many relations

This page introduces one-to-many relations and explains how to use them in your Prisma schema.

Overview

One-to-many (1-n) relations refer to relations where one record on one side of the relation can be connected to zero or more records on the other side. In the following example, there is one one-to-many relation between the User and Post models:

Relational databases
MongoDB
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
}

Note The posts field does not "manifest" in the underlying database schema. On the other side of the relation, the annotated relation field author and its relation scalar authorId represent the side of the relation that stores the foreign key in the underlying database.

This one-to-many relation expresses the following:

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

In the previous example, the author relation field of the Post model references the id field of the User model. You can also reference a different field. In this case, you need to mark the field with the @unique attribute, to guarantee that there is only a single User connected to each Post. In the following example, the author field references an email field in the User model, which is marked with the @unique attribute:

Relational databases
MongoDB
model User {
id Int @id @default(autoincrement())
email String @unique // <-- add unique attribute
posts Post[]
}
model Post {
id Int @id @default(autoincrement())
authorEmail String
author User @relation(fields: [authorEmail], references: [email])
}

In MySQL, you can create a foreign key with only an index on the referenced side, and not a unique constraint. In Prisma ORM versions 4.0.0 and later, if you introspect a relation of this type it will trigger a validation error. To fix this, you will need to add a @unique constraint to the referenced field.

Multi-field relations in relational databases

In relational databases only, you can also define this relation using multi-field IDs/composite key:

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 // relation scalar field (used in the `@relation` attribute above)
authorLastName String // relation scalar field (used in the `@relation` attribute above)
}

1-n relations in the database

Relational databases

The following example demonstrates how to create a 1-n relation in SQL:

CREATE TABLE "User" (
id SERIAL PRIMARY KEY
);
CREATE TABLE "Post" (
id SERIAL PRIMARY KEY,
"authorId" integer NOT NULL,
FOREIGN KEY ("authorId") REFERENCES "User"(id)
);

Since there's no UNIQUE constraint on the authorId column (the foreign key), you can create multiple Post records that point to the same User record. This makes the relation a one-to-many rather than a one-to-one.

The following example demonstrates how to create a 1-n relation in SQL using a composite key (firstName and lastName):

CREATE TABLE "User" (
firstName TEXT,
lastName TEXT,
PRIMARY KEY ("firstName","lastName")
);
CREATE TABLE "Post" (
id SERIAL PRIMARY KEY,
"authorFirstName" TEXT NOT NULL,
"authorLastName" TEXT NOT NULL,
FOREIGN KEY ("authorFirstName", "authorLastName") REFERENCES "User"("firstName", "lastName")
);

Comparing one-to-one and one-to-many relations

In relational databases, the main difference between a 1-1 and a 1-n-relation is that in a 1-1-relation the foreign key must have a UNIQUE constraint defined on it.

MongoDB

For MongoDB, Prisma ORM currently uses a , which means that documents reference each other by ID in a similar way to relational databases.

The following MongoDB document represents a User:

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

Each of the following Post MongoDB documents has an authorId field which references the same user:

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

Comparing one-to-one and one-to-many relations

In MongoDB, the only difference between a 1-1 and a 1-n is the number of documents referencing another document in the database - there are no constraints.

Required and optional relation fields in one-to-many relations

A 1-n-relation always has two relation fields:

The annotated relation field and relation scalar of a 1-n relation can either both be optional, or both be mandatory. On the other side of the relation, the list is always mandatory.

Optional one-to-many relation

In the following example, you can create a Post without assigning a User:

Relational databases
MongoDB
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 one-to-many relation

In the following example, you must assign a User when you create a Post:

Relational databases
MongoDB
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
}