type-merging-nullables

Nullable merges

This example demonstrates using nullable and not-null fields in merged data, as discussed in null records documentation.

This example demonstrates:

• Selecting nullability for merged fields.
• Returning nullable and not-nullable results.

Setup

cd type-merging-nullables

yarn install
yarn start

The following service is available for interactive queries:

• Stitched gateway: http://localhost:4000/graphql

For simplicity, all subservices in this example are run locally by the gateway server. You could easily break out any subservice into a standalone remote server following the combining local and remote schemas example.

Summary

This example explores the subtleties of nullable fields and returns within a stitched schema. The primary focus of this example is the Reviews service, where the User and Product types take slightly different approaches to the nullability of their respective reviews association. For context, start the gateway and try running this query:

query {
  users(ids: [2]) {
    username
    reviews {
      body
    }
  }
  products(upcs: [2]) {
    name
    reviews {
      body
    }
  }
}

This query fetches a user and a product, both without any associated reviews. You'll notice they handle this empty association in slightly different ways:

{
  "data": {
    "users": [
      {
        "username": "bigvader23",
        "reviews": []
      }
    ],
    "products": [
      {
        "name": "Toothbrush",
        "reviews": null
      }
    ]
  }
}

Not-null associations

First go into the Reviews service and compare its User type to the implementation of its _users query resolver:

schema:

type User {
  id: ID!
  reviews: [Review]!
}

query resolver:

_users: (root, { ids }) => ids.map(id => ({ id }))

In this implementation, any ID submitted to the _users query will be treated as a valid record and may resolve a result, for example:

query {
  _users(ids: ["DOES_NOT_EXIST"]) {
    id
    reviews { body }
  }
}

# --> [{ id: 'DOES_NOT_EXIST', reviews: [] }]

What's odd here is that the Reviews service is resolving a record for the "DOES_NOT_EXIST" ID on the blind assumption that it must exist somewhere else. In effect, the query is parroting all input into a legitimized result. Should it? That's entirely up to your own service architecture. One possible advantage of this pattern is that the not-null reviews:[Review]! association works because the service always guarentees a record with valid fields.

Nullable associations

Now go into the Reviews service and compare its Product type to the implementation of its _products query resolver:

schema:

type Product {
  upc: ID!
  reviews: [Review]
}

query resolver:

_products: (root, { upcs }) => upcs.map(upc => reviews.find(r => r.productUpc === upc) ? ({ upc }) : null)

In this implementation, only product UPCs that match a review in the database are treated as valid records. Unknown records simply return null without errors, for example:

query {
  _products(ids: ["DOES_NOT_EXIST"]) {
    id
    reviews { body }
  }
}

# --> [null]

From a pure service-oriented architecture perspective, this result makes a lot more sense because the Reviews service abstains from opinion on unknown IDs; it neither legitimizes them with a result, nor delegitimizes them with an error. The only caveat for this to work is that the reviews:[Review] association must be nullable because a value may not always be returned. The upc:ID! field may still be not-null because it is part of the Product type selectionSet (in index.js), and therefore will always be collected from other services.

Which approach is correct?

That depends. What are your requirements, and what makes the most sense? In general, erring on the side of returning null rather than a fabricated record is probably the more "pure" approach.