How do I do a left join in Prisma?

Vlad O.

Updated:

When working with databases, joining tables is a fundamental operation that allows you to combine rows from two or more tables based on a related column between them. A left join, one of the most common types of joins, returns all records from the left table and the matched records from the right table. If there is no match, the result is NULL on the side of the right table.

Prisma, an open-source database toolkit for TypeScript and Node.js, simplifies database operations with its ORM (Object-Relational Mapping) layer, allowing developers to write more intuitive and safer code. Implementing a left join in Prisma requires understanding how to use Prisma’s query methods to achieve the desired result, as Prisma does not use traditional SQL syntax but provides a rich set of APIs for database access and manipulation.

Implementing a Left Join in Prisma

Let’s say we have two tables: Users and Posts. Each user can have multiple posts, but each post is authored by only one user. Our goal is to fetch all users along with their posts. In SQL, this would typically involve a LEFT JOIN operation. However, in Prisma, we achieve this through relation queries.

Step 1: Define Your Models

First, ensure your Prisma schema models reflect the relationship between users and posts. Here’s an example schema definition:

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

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

In this schema, User and Post are related through posts and User fields, signifying a one-to-many relationship from User to Post.

Step 2: Fetching Users with Their Posts

To perform what is conceptually a left join to fetch all users along with their posts (if any), you use Prisma’s findMany method with the include option:

const prisma = new PrismaClient();

async function getUsersWithPosts() {
  const usersWithPosts = await prisma.user.findMany({
    include: {
      posts: true, // Include all posts for each user
    },
  });
  return usersWithPosts;
}
getUsersWithPosts().then(console.log).catch(console.error);

This code snippet fetches all users and their related posts. The include property tells Prisma to not only fetch the users but also to include data from the posts table where the authorId matches the id of the user. This is effectively how you perform a left join using Prisma, ensuring that even if a user has no posts, they will still be included in the result set.

Summary

While Prisma abstracts away traditional SQL join operations, it provides a powerful and intuitive API for managing and querying relational data. By leveraging the include option in your queries, you can easily perform operations akin to SQL joins, including the left join, ensuring your data is fetched in the most efficient and type-safe manner possible.

Prisma’s approach to data management not only makes code more readable but also significantly reduces the risk of errors, making your development process smoother and more efficient.

Posted in NodeJS tagged as orm prisma