Contributing fields to an entity

8. Contributing fields to an entity

15m

Overview

Even though we've defined the Listing entity in our reviews subgraph and contributed fields to it, we haven't told it how to resolve these fields.

In this lesson, we will:

Create a reference resolver
Populate the reviews and overallRating fields with data

The reference resolver

When we query for a particular listing and its review data, we need a way to tell the reviews subgraph which listing we're talking about. Then the reviews subgraph will understand what data to provide!

To do exactly this, the router passes along the entity representation we talked about in the last lesson: the minimum information required for the reviews subgraph to put together some idea of which listing it's fetching data for.

Example Listing entity representation

{
  "__typename": "Listing",
  "id": "listing-3"
}

But where in the reviews subgraph does this entity representation actually go?

This is just the piece we're missing: the reference resolver. The reference resolver is the special method that can receive an entity representation from the router, create a new instance of a Listing from that representation data, and return it.

Let's see what this looks like in code, and walk through the process step-by-step.

Adding `__resolveReference`

Back in the reviews subgraph, we need to tell our server what to do when it receives an entity representation for a particular listing. We'll do this by defining a special function, called __resolveReference, under a new entry in our resolvers map for Listing.

Open up reviews/src/resolvers.ts. Let's add the Listing key and the reference resolver as shown below.

reviews/src/resolvers.ts

// Query, Mutation entries above
Listing: {
  __resolveReference: () => {},
}

You'll see an error on this new function we've defined. This is because __resolveReference is not a known property on our Listing type as we've described it in the reviews schema.

Object literal may only specify known properties, and '__resolveReference'
does not exist in type 'ListingResolvers<any, Listing>'.

We can solve this by adding an additional property to our codegen.ts. Inside of the config object, we can add a new key called federation, and set its value to true. This lets our codegen process consider some of the federation-specific requirements of our resolvers: such as resolving entity representations received from the router!

codegen.ts

config: {
  contextType: "./context#DataSourceContext",
  federation: true
},

You might not see a change in your resolvers.ts file; but this time, TypeScript is upset that our function hasn't returned anything yet. Let's take care of that next.

We're going to receive the entity representation in our __resolveReference function as a parameter called representation. Then, we'll log it out, and return it.

reviews/src/resolvers.ts

__resolveReference: (representation) => {
  console.log(representation)
  return representation;
},

Even with this change, our function has yet another error. Here's what we'll see when we hover over __resolveReference.

Property 'reviews' is missing in type '{ __typename: "Listing"; }
& GraphQLRecursivePick<Listing, { id: true; }>' but required in type 'Listing'.

This long error is telling us one thing loud and clear: the "listing" that the __resolveReference function receives (an entity representation from the router) is missing some of the essential properties that we said it would have in our schema file. Essentially, our codegen process expects all of the Listing objects we work with in the resolvers.ts file to have all of the following fields.

type Listing @key(fields: "id") {
  id: ID!
  "The submitted reviews for this listing"
  reviews: [Review!]!
  "The overall calculated rating for a listing"
  overallRating: Float
}

The entity representation, however, consists of just two properties: __typename and id.

We need to fix the codegen process' idea of what a Listing looks like when it enters the __resolveReference function as an entity representation. To do this, we'll use a model.

Introducing models

The GraphQL schema defines how object types relate and how we get from one to the next. By moving from object to object, we can construct detailed queries that fetch everything we need in a single client request.

It's the job of the resolver functions to make this magic possible: they need the freedom to receive data that might not look like the types in our schema, and perform the logic needed to return the types we do expect.

A diagram showing data with different shapes entering a resolver, and data that conforms to the GraphqL schema leaving the resolver

To maintain type-safety, we need to clarify how our types of data differ between what resolvers receive (from data sources, or the router, as with our entity representation) and what resolvers return to clients.

In our case, our __resolveReference resolver thinks that it's going to receive an object of data that looks like our Listing GraphQL type; so we need to use a model to redefine its expectation.

Adding a `ListingEntityRepresentation` model

We'll create a new model that lets us more accurately describe the shape the listing entity representation takes. Then we'll integrate the model into our codegen process and resolve our type errors.

In the reviews/src directory, create a new file called models.ts.

📦 src
┣ 📂 datasources
┣ 📂 sequelize
┣ 📄 context.ts
┣ 📄 graphql.d.ts
┣ 📄 index.ts
┣ 📄 models.ts
┣ 📄 resolvers.ts
┣ 📄 schema.graphql
┗ 📄 types.ts

Inside, we'll define a basic model called ListingEntityRepresentation. We'll give it the single property we care about: id.

models.ts

export type ListingEntityRepresentation = {
  id: string;
};

Next, we'll add this model to our codegen process in codegen.ts.

Under the config key, we'll add another property called mappers. Here, we're able to specify a path to each model we want to be used as a mapper for a particular GraphQL type. This means that when codegen sees us working with a particular GraphQL object in our resolvers, it'll use the model that we provide as the reference for which properties the object should have.

codegen.ts

config: {
  contextType: "./context#DataSourceContext",
  federation: true,
  mappers: {
    Listing: "./models#ListingEntityRepresentation"
  }
},

Let's stop our running reviews server and restart it to make sure that the new codegen settings are applied. This should take care of our error!

Now let's try it out. Let's return to Sandbox and try out our query again.

query GetListingAndReviews {
  listing(id: "listing-1") {
    title
    description
    numOfBeds
    amenities {
      name
      category
    }
    overallRating
    reviews {
      id
      text
    }
  }
}

When we run the query, we still won't get any review-related data back; but when we check the terminal of our reviews subgraph, we'll see that our entity representation has arrived and is printed out!

{__typename=Listing, id=listing-1}

Our reviews subgraph is successfully receiving the listing representation from the router; this means we know which listing to provide data for. Now, we just need to define the resolvers for the fields that the reviews subgraph is responsible for: reviews and overallRating.

Adding `Listing.reviews`

Let's tackle the reviews method first.

Return to reviews/src/resolvers.ts. Let's clean up the console.log statement from __resolveReference. Then, just below __resolveReference, we'll add a new resolver for the Listing.reviews field.

reviews/src/resolvers.ts

Listing: {
  __resolveReference: (representation) => {
    return representation;
  },
  reviews: () => {}
}

This method should use the id from the entity representation to find and return all the relevant reviews in the database. Because this resolver resolves a field on an entity type, we know that the previous resolver in the chain was the __resolveReference resolver we just defined. This means we can access its return value (the Listing instance) using parent, the first positional argument each resolver receives.

Let's destructure parent for the id property.

reviews/src/resolvers.ts

reviews: ({ id }) => {
  // TODO
},

Our resolvers already have access to a property on the server's dataSources called reviewsDb, which is a class that provides a number of methods that operate on the underlying reviews database. Let's destructure our resolver's third positional argument, contextValue, for the dataSources property.

reviews/src/resolvers.ts

reviews: ({ id }, _, { dataSources }) => {
  // TODO
},

With our listing id in hand, we can look up all of the reviews in our database that are associated with that listing. The reviewsDb class instance provides a method to look up reviews by a specific listing ID, called getReviewsByListing. Let's call this method, passing in the id property we got from our parent argument.

reviews/src/resolvers.ts

reviews: ({ id }, _, { dataSources }) => {
  return dataSources.reviewsDb.getReviewsByListing(id);
},

The `overallRating` method

Onto the overallRating resolver!

Try this one out on your own. (Hint: check out the datasources/reviews.ts file for a helpful method!)

When you're ready, compare your code to our finished resolver function below.

The overallRating resolver

overallRating: ({ id }, _, { dataSources }) => {
  return dataSources.reviewsDb.getOverallRatingForListing(id);
},

Running our dream query

With the rover dev process still running, let's try out our query at http://localhost:4002.

query GetListingAndReviews {
  listing(id: "listing-1") {
    title
    description
    numOfBeds
    amenities {
      name
      category
    }
    overallRating
    reviews {
      id
      text
    }
  }
}

Submit the query, and... we've got data! 🎉 We've associated reviews with a listing and made our dream query come to life!

{
  "data": {
    "listing": {
      "title": "Cave campsite in snowy MoundiiX",
      "description": "Enjoy this amazing cave campsite in snow MoundiiX, where you'll be one with the nature and wildlife in this wintery planet. All space survival amenities are available. We have complementary dehydrated wine upon your arrival. Check in between 34:00 and 72:00. The nearest village is 3AU away, so please plan accordingly. Recommended for extreme outdoor adventurers.",
      "numOfBeds": 2,
      "amenities": [
        {
          "name": "Towel",
          "category": "Accommodation Details"
        },
        {
          "name": "Oxygen",
          "category": "Space Survival"
        },
        {
          "name": "Prepackaged meals",
          "category": "Space Survival"
        },
        {
          "name": "SOS button",
          "category": "Space Survival"
        },
        {
          "name": "Meteor shower shield",
          "category": "Space Survival"
        },
        {
          "name": "First-aid kit",
          "category": "Space Survival"
        },
        {
          "name": "Water recycler",
          "category": "Space Survival"
        },
        {
          "name": "Panic button",
          "category": "Space Survival"
        },
        {
          "name": "Emergency life support systems",
          "category": "Space Survival"
        },
        {
          "name": "Universal translator",
          "category": "Space Survival"
        },
        {
          "name": "Aquatic breathing aid",
          "category": "Space Survival"
        },
        {
          "name": "Acid lake access",
          "category": "Outdoors"
        },
        {
          "name": "Time travel paradoxes",
          "category": "Outdoors"
        },
        {
          "name": "Meteor showers",
          "category": "Outdoors"
        },
        {
          "name": "Wildlife",
          "category": "Outdoors"
        }
      ],
      "overallRating": 3.75,
      "reviews": [
        {
          "id": "review-1",
          "text": "Wow, what an experience! I've never stayed in a cave before, so I was a little unprepared. Luckily, this listing had all the amenities I needed to feel safe and prepared for anything. Seriously, great experience. Giving 4 stars because the flies kept coming in at night through a hole in the cave mesh entrance."
        },
        {
          "id": "review-2",
          "text": "100% enjoyed the wilderness experience. Do not book if you are not an adventurer and lover of the outdoors."
        },
        {
          "id": "review-3",
          "text": "I thought this was going to be a cozy cave, but I was sorely disappointed. The mattress was hard, I could feel stones digging into my back. And it was COLD. They need to be more clear about this on the description."
        },
        {
          "id": "review-12",
          "text": "Description was accurate. It was indeed a cave campsite in the snowy part of the planet. Exactly what I needed."
        }
      ]
    }
  }
}

Reviewing the query plan

Let's check out the query plan to see how this data came together. We'll see that first, the router will fetch data from listings, and then use that data to build out its request to reviews. The last step is flattening the response from both subgraphs into a single instance of a Listing type for the listing field we've queried!

http://localhost:4002

The operation in Sandbox, with the Query Plan Preview opened, showing a linear path to retrieve data from both subgraphs

Still seeing reviews: null? Try restarting your reviews server! The rover dev process running our router on port 4002 will automatically refresh.

Earlier, we stated that one of the requirements for a subgraph that contributes fields to an entity is that it also defines a reference resolver.

We took care of this in the reviews subgraph... but what about listings?

Well, we've gotten away with not writing a reference resolver in the listings subgraph for a simple reason: in its current state, our API never requires the router to send an entity representation to the listings subgraph.

Here's why. Whether we're building a query for featuredListings or a single listing, those entrypoints exist in the listings subgraph; so this subgraph already has an idea of which listing it's resolving data for.

Let's imagine instead that we update our reviews subgraph. Maybe we give our Review type a listing field that points back to the Listing it was written for.

A theoretical implementation of Review

type Review {
  # ... other Review fields
  listing: Listing!
}

Now let's say we use the Query entrypoint provided by our reviews subgraph, allReviews, to build the query below.

A query for all reviews, and their listings

query GetAllReviews {
  allReviews {
    id
    text
    listing {
      title
    }
  }
}

We can walk through exactly how this query should be resolved. Because the Query.allReviews field is provided by the reviews subgraph, that's where we'd start. For each Review type returned by the allReviews field, we'd then dig deeper to resolve its listing field. Here's where our router would need to take the Listing entity representation from the reviews subgraph, and pass it over to the listings subgraph, in order to resolve the title field!

Example Listing entity representation

{
  "__typename": "Listing",
  "id": "listing-3"
}

Now we'd run into trouble. The listings subgraph doesn't have a method that can receive this entity representation. We haven't added one!

Even though the scenario we just ran through doesn't actually apply to our API, we shouldn't rule out the possibility that one day our needs will change. In that case, our listings subgraph will need to know how to resolve a Listing entity representation that comes from somewhere else!

Practice

Where should an entity's reference resolver method be defined?

In every subgraph that is composed into the supergraph.In each subgraph that contributes fields to the entity.In the router.In the first subgraph that defines the entity.

Key takeaways

Any subgraph that contributes fields to an entity needs to define a reference resolver method for that entity. This method is called whenever the router needs to access fields of the entity from within another subgraph.
An entity representation is an object that the router uses to represent a specific instance of an entity. It includes the entity's type and its key field(s).
The __resolveReference function receives an entity representation from the router for a specific GraphQL type. This representation contains all the data a subgraph needs to understand how to populate data for the fields it contributes.

Up next

Awesome! Our listings and reviews services are now collaborating on the same Listing entity. Each subgraph contributes its own fields, and the router launched by our local rover dev process packages up the response for us. Emphasis on the word local; to get our changes actually "live" (at least in the tutorial sense of the word), we need to tell GraphOS about them!

In the next lesson, we'll take a look at how we can land these changes safely and confidently using schema checks and launches.

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Overview

The reference resolver

Adding __resolveReference

Introducing models

Adding a ListingEntityRepresentation model

Adding Listing.reviews

The overallRating method

Running our dream query

Reviewing the query plan

Practice

Key takeaways

Up next

Share your questions and comments about this lesson

Adding `__resolveReference`

Adding a `ListingEntityRepresentation` model

Adding `Listing.reviews`

The `overallRating` method