Contributing to an entity

13. Contributing to an entity

Overview

In the previous lesson, we just saw how subgraphs can reference an entity as a field's return type. Now, let's take a look at how subgraphs can contribute fields to an entity.

In this lesson, we will:

Learn how multiple subgraphs can contribute fields to an entity
Update the Location entity in our reviews subgraph schema by contributing the reviewsForLocation and overallRating fields

The FlyBy schema diagram, with pencil icons next to the Location.reviews and Location.overallRating fields

✏️ Contributing fields

Remembering our FlyBy UI, we know it needs to fetch each location's overallRating, along with a list of its reviewsForLocation:

A query containing fields that will be populated by our two subgraphs, locations and reviews

By following the separation of concerns principle, it makes sense that any data about ratings or reviews is populated by the reviews subgraph, so let's head on over there and make those additions!

Open up the subgraph-reviews/reviews.graphql file.
Find the Location entity definition in the schema. We previously defined this as a stub of the Location type.
subgraph-reviews/reviews.graphql
type Location @key(fields: "id", resolvable: false) {
id: ID!
}
By default, a subgraph should only contribute fields that aren't defined by other subgraphs, with the exception of the primary key field. This means that because the locations subgraph defines name, description, and photo as fields for the Location type, we won't - and shouldn't - define those fields here in the reviews subgraph!
Note: You can override the default behavior explained above to allow multiple subgraphs to resolve the same field by applying either the @shareable or @provides directive. This is an optional performance optimization that can instruct the router on how to plan the execution of a query across as few subgraphs as possible.
Because we now want the reviews subgraph to contribute new fields to the Location definition, the first thing we need to do is remove the resolvable: false property from the @key directive. This will enable our reviews subgraph to define and resolve its own Location fields.
Remove the resolvable: false property from the Location type's @key directive.
subgraph-reviews/reviews.graphql
type Location @key(fields: "id") {
id: ID!
}

Task!

I've removed the resolvable: false property from the @key directive.

✏️ Adding new fields to the Location entity

Now we're ready to add the two new fields.

the overallRating field, which returns a Float
the reviewsForLocation field, which returns a non-null list of Review objects.

We'll also add descriptions to these fields so we can quickly see what they represent.

subgraph-reviews/reviews.graphql

type Location @key(fields: "id") {
  id: ID!
  "The calculated overall rating based on all reviews"
  overallRating: Float
  "All submitted reviews about this location"
  reviewsForLocation: [Review]!
}

✏️ Adding resolvers

Each of these fields needs a resolver function to return data, so let's take care of that next.

Open the resolvers.js file in the subgraph-reviews directory.

Add a Location entry to the resolvers map. We'll also add two empty resolver functions for the overallRating and reviewsForLocation fields.

subgraph-reviews/resolvers.js

const resolvers = {
  Query: {
    // ...
  },
  Location: {
    overallRating: () => {},
    reviewsForLocation: () => {},
  },
  Review: {
    // ...
  },
  Mutation: {
    // ...
  },
};

We'll start with the overallRating resolver. First, we'll destructure the parent argument (a Location object) to get the id field. We'll also destructure the contextValue argument to pull out our dataSources.
subgraph-reviews/resolvers.js
overallRating: ({id}, _, {dataSources}) => {
// TODO
},
Resolver functions have a specific signature with four optional parameters: parent, args, contextValue, and info.
- parent:
  parent is the returned value of the resolver for this field's parent.
- args:
  args is an object that contains all GraphQL arguments that were provided for the field by the GraphQL operation.
- contextValue:
  contextValue is an object shared across all resolvers that are executing for a particular operation.
- info:
  info contains information about the operation's execution state, including the field name, the path to the field from the root, and more.
You can learn more about resolver parameters in action by checking out our Odyssey Lift-Off II course!
Inside the function, we'll return the results of calling our dataSources object, its ReviewsAPI, and its getOverallRatingForLocation method. Then, pass in the id of the location that we're querying.
subgraph-reviews/resolvers.js
overallRating: ({id}, _, {dataSources}) => {
return dataSources.reviewsAPI.getOverallRatingForLocation(id);
},
Note: You can check out how the getOverallRatingForLocation method works by peeking inside the subgraph-reviews/datasources/ReviewsApi.js file.
Next, we'll set up the resolver function for the reviewsForLocation field and follow the same structure as before. This time, we'll use the getReviewsForLocation method of the ReviewsAPI to fetch all reviews for a location based on its id.
subgraph-reviews/resolvers.js
reviewsForLocation: ({id}, _, {dataSources}) => {
return dataSources.reviewsAPI.getReviewsForLocation(id);
},

Wonderful! Our resolvers receive a location's id and can return the right data for that location.

Adding a `__resolveReference` function

Earlier, we learned that each subgraph that contributes fields to an entity needs to define a reference resolver for that entity.

We already defined the reference resolver in the locations subgraph, but the reviews subgraph also needs some way of knowing which particular location object it's resolving fields for.

Here's the good news: because we're using Apollo Server, defining the reference resolver function explicitly in the reviews subgraph is not a requirement. Apollo Server defines a default reference resolver for any entities we don't define one for.

This diagram shows how the __resolveReference function works by default with a query for a particular Location.

The reviews subgraph resolve reference function called for a particular queried Location

A queried location is resolved in the locations subgraph based on its id argument.
When the server reaches the reviewsForLocation field, the router knows that this is the responsibility of the reviews subgraph. The __resolveReference function receives the queried Location object that the locations subgraph returned.
The reviewsForLocation resolver receives the referenced Location object as its parent argument, which it can then destructure and use to resolve data.

Even with this feature working for us under the hood, we'll walk through the steps to add this function ourselves in the optional section below, and review what happens when our router associates data across subgraphs.

Let's use the query below as an example of how the reference resolver function comes into play. We'll walk through the steps the router will follow to build a query plan, and execute this query.

query GetLocationReviewData {
  locations {
    name
    overallRating
    reviewsForLocation {
      comment
    }
  }
}

Adding the reference resolver

Open up the subgraph-reviews/resolvers.js file and add a new function called __resolveReference.

subgraph-reviews/resolvers.js

const resolvers = {
  Query: {
    // ...
  },
  Location: {
    __resolveReference: () => {
      // TODO
    }
    overallRating: ({id}, _, {dataSources}) => {
      return dataSources.reviewsAPI.getOverallRatingForLocation(id);
    },
    reviewsForLocation: ({id}, _, {dataSources}) => {
      return dataSources.reviewsAPI.getReviewsForLocation(id);
    }
  },
  Review: {
    // ...
  },
  Mutation: {
    // ...
  }
};

This function is where our reviews subgraph will receive an entity representation passed by any other subgraph referencing the Location entity.

The Query Plan

The query starts with the locations entry point into our schema, which is defined in the locations subgraph. The router will start with a request to this subgraph.

The router encounters the name field, and checks this against the supergraph schema. This field is added to the query plan as resolvable from the locations subgraph.

On the next line, the router encounters the overallRating field. From the supergraph schema, the router sees that this field can only be resolved by the reviews subgraph.

The router adds a new operation to the query plan, this time requesting the overallRating for each location from the reviews subgraph.

The next field, reviewsForLocation, along with its subfield, comment, are likewise added to the request to the reviews subgraph.

To query the reviews subgraph, the router asks the locations subgraph for some extra information about the locations field: each location's __typename and id, which make up the location's entity representation.

Executing the query plan

The router begins carrying out the query plan outlined above, starting with asking the locations subgraph for data, including the representations for each of the objects in the queried locations.

The locations subgraph resolves all of the fields requested from it as it normally would, and returns the representations for each of the locations.

The router uses the _entities field to build a request for the reviews subgraph, which takes in the locations representations as an argument. The router also specifies the fields it needs from the reviews subgraph.

The __resolveReference function defined for the Location entity in the reviews subgraph is called with each representation object, which contains the __typename and id properties.

Because the __typename property will identify each representation as type Location, we can return this reference exactly as it is, and the overallRating and reviewsForLocation resolvers will receive the passed id in their parent argument when they are queried.

subgraph-reviews/resolvers.js

Location: {
  __resolveReference: (location) => {
    return location;
  }
  overallRating: ({id}, _, {dataSources}) => {
    return dataSources.reviewsAPI.getOverallRatingForLocation(id);
  },
  reviewsForLocation: ({id}, _, {dataSources}) => {
    return dataSources.reviewsAPI.getReviewsForLocation(id);
  }
},

This is the default reference resolver that Apollo Server takes care of defining for us, behind the scenes!

When the reviews subgraph has gathered all of the data for each representation object, it combines it and sends it back to the router as the return value of the _entities field.

So are we ready to put our supergraph to the test? Not so fast! Remember we made schema changes! These changes need to be published to the registry, or we'll run into the same problem we faced in the last lesson.

So we'll run the rover subgraph publish command, passing it the values for our reviews subgraph.

rover subgraph publish <APOLLO_GRAPH_REF> \
  --name reviews \
  --schema ./subgraph-reviews/reviews.graphql

Querying data across subgraphs

With a successful publish, let's return to Studio and refresh the Explorer. We can see that our list of subfields now includes overallRating and reviewsForLocation!

studio.apollographql.com/graph/flyby/explorer?variant=current

Let's include these fields in a new query to our router. We'll use the query the client needs for the location details page.

query GetLocationDetails($locationId: ID!) {
  location(id: $locationId) {
    id
    name
    description
    photo
    overallRating
    reviewsForLocation {
      id
      comment
      rating
    }
  }
}

In the Variables panel:

{ "locationId": "loc-1" }

Look at this sweet sweet data!

{
  "data": {
    "locations": [
      {
        "id": "loc-1",
        "name": "The Living Ocean of New Lemuria",
        "photo": "https://res.cloudinary.com/apollographql/image/upload/v1644381344/odyssey/federation-course1/FlyBy%20illustrations/Landscape_4_lkmvlw.png",
        "description": "Surviving is usually extremely difficult, especially when nutrients are scarce and you have to choose between growing or reproducing. One species on this planet has developed a nifty method to prepare for this. Once full matured, this species will split into 2 versions of itself and attached to each other, so it's essentially reproducing. Once those 2 are fully grown, they newly grown version will either detach itself if enough nutrients are available or it becomes a storage unit for the original, if nutrients are scarce. If nutrients continue to be scarce, the original will use slowly consume the nutrients in the new version in the hope that new nutrients become available again and it can repeat the cycle.",
        "overallRating": 4.25,
        "reviewsForLocation": [
          {
            "id": "rev-1",
            "comment": "I would also like to say thank you to all your staff! I would gladly pay over 600 dollars for planet. Planet was worth a fortune to my company. After using planet my business skyrocketed!",
            "rating": 5
          },
          {
            "id": "rev-2",
            "comment": "It's really wonderful. We have no regrets! Keep up the excellent work.",
            "rating": 5
          },
          {
            "id": "rev-3",
            "comment": "This is simply unbelievable! It's the perfect solution for our business. Really good. I don't always clop, but when I do, it's because of planet",
            "rating": 5
          },
          {
            "id": "rev-4",
            "comment": "Planet is exactly what our business has been lacking. It's incredible. If you want real marketing that works and effective implementation - planet's got you covered.",
            "rating": 2
          }
        ]
      },
      {
        "id": "loc-2",
        "name": "Vinci",
        "photo": "https://res.cloudinary.com/apollographql/image/upload/v1644381349/odyssey/federation-course1/FlyBy%20illustrations/Landscape_15_tiqel5.png",
        "description": "Many of the creatures on this planet have evolved into gliders, so to speak. Most of the fish and aquatic mammals, despite coming in various shapes and sizes, tend to glide through the water without effort, similar to how manta's glide on Earth. However, the surface species are more astonishing. Similar to the flying squirrels or the vultures of Earth, many of the species on this planet have developed ways to effortlessly move from one place to another by using the winds. But there is one species which shows signs of sentience. These species, a type of bird, love to play and have become masters of flight. Similar to how dolphins play, explore and learn, these species use their intellect and courage to play and sometimes challenge each other to death defying tricks.",
        "overallRating": 3.5,
        "reviewsForLocation": [
          {
            "id": "rev-5",
            "comment": "Thanks planet! I was amazed at the quality of planet. Planet did exactly what you said it does.",
            "rating": 4
          },
          {
            "id": "rev-6",
            "comment": "I would also like to say thank you to all your staff. I would gladly pay over 600 dollars for planet. Planet was worth a fortune to my company. After using planet my business skyrocketed!",
            "rating": 3
          },
          {
            "id": "rev-7",
            "comment": "It's really wonderful. We have no regrets! Keep up the excellent work.",
            "rating": 2
          },
          {
            "id": "rev-8",
            "comment": "This is simply unbelievable! It's the perfect solution for our business. Really good. I don't always clop, but when I do, it's because of planet",
            "rating": 5
          }
        ]
      },
      {
        "id": "loc-3",
        "name": "Asteroid B-612",
        "photo": "https://res.cloudinary.com/apollographql/image/upload/v1644381343/odyssey/federation-course1/FlyBy%20illustrations/Landscape_6_vt6y3v.png",
        "description": "Nutrients are always needed but not always around, so organisms have to find ways to get them. Common ways are using different roots to find them in deep or shallow grounds or even stealing them from others, but on this planet many species have found a different balance. Unlike most plants on Earth who tend to only produce oxygen and nutrients, usually in the form of sugars, for itself, the organisms on this planet also produce other forms of nutrients for itself, usually for different purposes. These processes often lead to many byproducts which it doesn't need and are thus discarded. These discarded products are exactly what other species need to live and in turn produce byproducts it discards for the other organisms, leading to a delicate balance.",
        "overallRating": 5,
        "reviewsForLocation": [
          {
            "id": "rev-9",
            "comment": "Planet is exactly what our business has been lacking. It's incredible. If you want real marketing that works and effective implementation - planet's got you covered.",
            "rating": 5
          }
        ]
      },
      {
        "id": "loc-4",
        "name": "Krypton",
        "photo": "https://res.cloudinary.com/apollographql/image/upload/v1644381344/odyssey/federation-course1/FlyBy%20illustrations/Landscape_9_kbenjj.png",
        "description": "Similar to the surface, the underwater world has little more to offer than basic lifeforms. However, this planet has an astonishing water world. Almost everything is covered in a type of sea-grass. This grass varies in length depending on the region, but they're all part of the same species. But what's probably more surprising are the 'flowers' you'll find in these fields of sea-grass. These flowers can only be described as primitive soft corals, but they're neither coral nor plant.",
        "overallRating": 5,
        "reviewsForLocation": [
          {
            "id": "rev-10",
            "comment": "Thanks planet! I was amazed at the quality of planet. Planet did exactly what you said it does.",
            "rating": 5
          }
        ]
      },
      {
        "id": "loc-5",
        "name": "Zenn-la",
        "photo": "https://res.cloudinary.com/apollographql/image/upload/v1644381346/odyssey/federation-course1/FlyBy%20illustrations/Landscape_8_zd1e68.png",
        "description": "The plant-like organisms on this planet are made up of millions of flowers. Their combined colors and scents make for an amazing spectacle, but they leave little space for other species, which is why there are only very few bush and shrub species. Fungi, grasses and trees are non-existent.",
        "overallRating": 0,
        "reviewsForLocation": []
      }
    ]
  }
}

The client is going to be thrilled that they're getting information about a location from both subgraphs without having to do any work to put it all together themselves!

Even better, we didn't have to restart our router. This is thanks to our router's connection to Apollo Uplink. Our newly published subgraph triggered Apollo Studio to compose a new supergraph schema. Our router then polled the Uplink and fetched the new supergraph schema. Best of all, the router started to use the new supergraph schema immediately, enabling us to query the new fields right away!

With that, we can finally check off the last two fields of our schema agreement!

The FlyBy schema diagram, with all the fields checked off!

Practice

Check your understanding!

In a federated graph, we should define our subgraphs based on

instead of

. Types containing fields that can be resolved across multiple subgraphs are called

. These types always have a

that enables different subgraphs to associate data with the same object. To keep its supergraph schema up to date, our

can poll the

Drag items from this box to the blanks above

assignments
router
key field
types
create
concerns
delete
endpoints
entities
Uplink
search

Code Challenge!

You're working on a federated graph that manages book information. The authors subgraph defines an Author entity with a primary key field id of non-nullable type ID. You want to use the Author entity in the books subgraph. Define the Author entity below, and add a new field, books, which returns a non-nullable list of non-nullable type Book.

Key takeaways

A subgraph that contributes fields to an entity should define the following:
- The entity, using the @key directive and its primary key fields, as well as the new fields the subgraph defines
- A __resolveReference function to know which particular entity instance a subgraph is resolving fields for. This can be taken care of by default by Apollo Server.
A federated architecture helps organize and illustrate the relationships between types across our graph in a way that an app developer (or multiple teams of developers!) would want to consume the data.
When both subgraphs use the same primary key to associate data for a type, the router coordinates data from both sources and bundles it up in a single response.

Up next

Congratulations, you've finished implementing all the fields in the FlyBy graph!

In the next and final lesson, we'll put the backend and frontend together and finally see FlyBy working in a browser!

Overview

✏️ Contributing fields

✏️ Adding new fields to the Location entity

✏️ Adding resolvers

Adding a __resolveReference function

Adding the reference resolver

The Query Plan

Executing the query plan

Querying data across subgraphs

Practice

Key takeaways

Up next

Adding a `__resolveReference` function