While working on a project using Prisma with MongoDB, I encountered some confusion about how Prisma handles relationships between data entities. Specifically, I was puzzled about the role of the connect operation and the arrays in the models representing relationships. Despite creating related data, these arrays weren’t automatically updated, leading to misunderstandings about how to fetch related data. In this blog post, I aim to demystify these aspects using a simple example of Users and Posts

Consider a blogging application where you have Users and Posts. Each User can have multiple Posts, but each Post belongs to one User. This is a one-to-many relationship.

In your Prisma schema, you might represent this relationship like this:

model User {
  id    String @id @default(auto()) @map("_id") @db.ObjectId
  name  String
  posts Post[]
}

model Post {
  id     String @id @default(auto()) @map("_id") @db.ObjectId
  title  String
  body   String
  userId String @db.ObjectId
  user   User   @relation(fields: [userId], references: [id])
}

Here, the posts array in the User model is intended to hold references (ObjectIds) to the Post documents associated with a User.

Now, let’s say you create a new Post for a User:

JavaScript

const post = await prisma.post.create({
  data: {
    title: "My First Post",
    body: "This is my first post.",
    user: {
      connect: {
        id: "<ObjectId of the User>",
      },
    },
  },
});

AI-generated code. Review and use carefully. More info on FAQ.

In this code, "<ObjectId of the User>" should be replaced with the actual ObjectId of the User. The connect operation tells Prisma to establish a relationship between the new Post and the User by storing the ObjectId of the User in the userId field of the Post.

However, this connect operation does not automatically update the posts array in the User document to include the ObjectId of the new Post. The posts array in the User document would still be empty after this operation.

Despite this, you can still fetch a User and all their associated Post documents using Prisma’s findUnique method with the include option:

JavaScript

const userWithPosts = await prisma.user.findUnique({
  where: { id: "<ObjectId of the User>" },
  include: { posts: true },
});

In this code, "<ObjectId of the User>" should be replaced with the actual ObjectId of the User. The returned userWithPosts object will include all the Post documents associated with the User, even though the posts array in the User document is empty.

This is because each Post document contains a userId field, which is a reference to the User document it’s associated with. Prisma can use this field to find all Post documents associated with a User.