Exercise: Defining & contributing to an entity

7. Exercise: Defining & contributing to an entity

Exercise: Defining & contributing to an entity

🎯 Goal: Query for a listing's title, overallRating and reviews information.

Open up Explorer. When you run the operation below, you'll get an error!

query GetListingDetails {
  listing(id: "listing-1") {
    title
    overallRating
    reviews {
      rating
      text
    }
  }
}

After successfully completing the exercise, you should get data back when running the GetListingDetails operation again.

{
  "data": {
    "listing": {
      "title": "Cave campsite in snowy MoundiiX",
      "overallRating": 3.75,
      "reviews": [
        {
          "rating": 4,
          "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."
        },
        {
          "rating": 5,
          "text": "100% enjoyed the wilderness experience. Do not book if you are not an adventurer and lover of the outdoors."
        },
        {
          "rating": 1,
          "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."
        },
        {
          "rating": 5,
          "text": "Description was accurate. It was indeed a cave campsite in the snowy part of the planet. Exactly what I needed."
        }
      ]
    }
  }
}

Hints

First, define the entity (remember, you need two things to do this!).

Then, contribute the two fields (overallRating and reviews) for the entity in the schema. Write separate resolver functions for each field.

In the reviews subgraph schema, we need to add the Listing type as an entity. What two things do we need to define an entity?

- A @key directive
- A reference resolver

What is the signature of the reference resolver function? What should it return?

subgraph-reviews/src/resolvers.js

Listing: {
  __resolveReference(representation, context, info) {
    // return a Listing object with fields the reviews subgraph can contribute
    // you can start with just returning an object with the id field.
  }
}

Note: You can return the representation directly and use a resolver chain to resolve the additional fields (such as Listing.overallRating and Listing.reviews)

Add the two new fields to the schema. Each field will have its own resolver (go to subgraph-reviews/src/resolvers.js and find the boilerplate code there).

Use the following data source methods to retrieve the data you need:

Listing.overallRating: dataSources.reviewsDb.getOverallRatingForListing(id)
Listing.reviews: dataSources.reviewsDb.getReviewsByListing(id)

✅ Solution

Expand the sections below for a quick copy-paste, or follow the instructions to go step-by-step.

extend schema
  @link(url: "https://specs.apollo.dev/federation/v2.7", import: ["@key"])

type Query {
  "A list of the most recent reviews in the database"
  recentReviews: [Review!]!
}

"A comment and a rating submitted by a guest about a particular listing"
type Review {
  id: ID!
  "Comment the author has written"
  text: String!
  "The numerical rating for the review target, on a scale of 1-5, with 5 being excellent."
  rating: Float!
}

type Listing @key(fields: "id") {
  id: ID!
  overallRating: Float
  reviews: [Review!]!
}

export const resolvers = {
  Query: {
    recentReviews: async (_, __, { dataSources }) => {
      return dataSources.reviewsDb.getMostRecentReviews();
    },
  },
  Listing: {
    // Exercise: Contributing to an entity
    __resolveReference: (listing) => {
      return listing;
    },
    overallRating: ({ id }, __, { dataSources }) => {
      return dataSources.reviewsDb.getOverallRatingForListing(id);
    },
    reviews: ({ id }, __, { dataSources }) => {
      return dataSources.reviewsDb.getReviewsByListing(id);
    },
    // Exercise: Using requires/external
    moneyValueGuaranteed: (listing, _, { dataSources }) => {
      // Use this function: dataSources.reviewsDb.calculateMoneyValueGuarantee(
      //   listing.costPerNight,
      //   listing.id
      // )
    },
  },
  // Exercise: Reference an entity
  Review: {
    listing: () => {
      // Hint: Use the parent parameter
      // Hint: What fields do you need to return an entity representation for a Listing?
    },
  },
};

In subgraph-reviews/reviews.graphql, tag the Listing type as an entity using @key with the id as the key field.
subgraph-reviews/reviews.graphql
type Listing @key(fields: "id") {
id: ID!
}

Add a Listing reference resolver in subgraph-reviews/src/resolvers.js.

subgraph-reviews/src/resolvers.js

Listing: {
  __resolveReference(listing) {
   // listing looks something like this { id: "listing-1"}
   // console.log(listing);
    return listing;
  },
}

Add two fields: overallRating that returns a Float and reviews that returns an list of Review types.

subgraph-reviews/reviews.graphql

type Listing @key(fields: "id") {
  id: ID!
  overallRating: Float
  reviews: [Review!]!
}

Add a resolver function for Listing.overallRating.

subgraph-reviews/src/resolvers.js

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

Add a resolver for Listing.reviews.

subgraph-reviews/src/resolvers.js

Listing: {
  __resolveReference(listing) {
   return listing;
  },
  overallRating: ({ id }, _, { dataSources }) => {
    return dataSources.reviewsDb.getOverallRatingForListing(id);
  },
  reviews: ({ id }, _, { dataSources }) => {
    return dataSources.reviewsDb.getReviewsByListing(id);
  },
},

BONUS: Examine the query plan

Here's the operation we're running:

query GetListingDetails {
  listing(id: "listing-1") {
    title
    overallRating
    reviews {
      rating
      text
    }
  }
}

Examine the query plan for the operation.

In the query plan, which two fields is the router asking for that are not included in the original GraphQL operation?

__typenameserviceidcostPerNightdescription

Why are those two fields included in the query plan?

The listings subgraph automatically includes them in every operation.The reviews subgraph needs those fields to calculate the overallRating.The router needs those fields for the entity representation.The client asked for those fields.

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