Prisma Datamodel: Primary key as a combination of two relational models

Question

I have a problem in Prisma data modeling where I have to constrain that a user can submit only one review for a product. I have following design for the non-constrained situation.

Should Customer and Product be combined into a primary key in ProductReview model, or should this constraint be imposed at the application server level, and not at the database level?

Datamodel for now (non-constrained version):

type Product {
  id: ID! @unique
  title: String!
  reviews: [ProductReview!]! @relation(name: "ProductReviews", onDelete: CASCADE)
}

type Customer {
  id: ID! @unique
  email: String @unique
}

type ProductReview {
  id: ID! @unique
  forProduct: Product! @relation(name: "ProductReviews", onDelete: SET_NULL)
  byCustomer: Customer!
  review: String!
  ratinng: Float!
}

Answer 1

It looks like Prisma v2 introduced composite primary keys:

https://newreleases.io/project/github/prisma/prisma/release/2.0.0-preview023

An example from that link:

model User {
  firstName String
  lastName  String
  email     String

  @@id([firstName, lastName])
}

So in the given question example, it should be possible to add to ProductReview:

@@id([id, forProduct])

Answer 2

I have to constrain that a user can submit only one review for a product. I have following design for the non-constrained situation.

Unfortunately, this is currently not possible with Prisma. There already is an open feature request asking for this functionality, please leave your 👍 on the issue!

To get that functionality in your application, you'll need to implement that constraint manually on the application layer (e.g. express, apollo-server or graphql-yoga).

You can take a look at this page of How to GraphQL where there's a similar situation with the User, Link and Vote types. Here's how the resolver to create a Vote and ensurs no votes from that user already exist is implemented with graphql-yoga:

async function vote(parent, args, context, info) {
  // 1
  const userId = getUserId(context)

  // 2
  const linkExists = await context.db.exists.Vote({
    user: { id: userId },
    link: { id: args.linkId },
  })
  if (linkExists) {
    throw new Error(`Already voted for link: ${args.linkId}`)
  }

  // 3
  return context.db.mutation.createVote(
    {
      data: {
        user: { connect: { id: userId } },
        link: { connect: { id: args.linkId } },
      },
    },
    info,
  )
}