5. Resolver chains
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📥 Retrieving module details

To retrieve the module details for a given track, we'll need to use the GET track/:id/modules endpoint in our REST API.

Let's try out the endpoint, giving it our track ID c_0 again.

The response we get is an array of modules with the details that we need.

Screenshot of REST API docs with focus on the GET track/:id/modules endpoint

Our track page needs the id, title and length for now. Let's update our TrackAPI to call this endpoint.

💾 Updating the RESTDataSource

In the server/src/datasources folder, open up the track-api.js file.

Let's create a method called getTrackModules. It takes a trackId as a parameter. Inside, it'll make a get call to the track/${trackId}/modules endpoint and return the results.

getTrackModules(trackId) {
  return this.get(`track/${trackId}/modules`);
}

Now we can use this new datasource method in our . Let's hop back to the resolvers.js file in the server/src folder.

First let's determine where exactly we can place our call to get the details of a track's modules. We know that we need that information in our track . But should we add the call here in the track ?

// EXAMPLE ONLY - should we add the getTrackModules call here in the track resolver?
track: async (_, { id }, { dataSources }) => {
  // get track details
  const track = dataSources.trackAPI.getTrack(id);


  // get module details for the track
  const modules = await dataSources.trackAPI.getTrackModules(id);


  // shape the data in the way that the schema expects it
  return { ...track, modules };
};

We certainly could. This is a similar problem to the one we faced in Lift-off II, when we were trying to figure out where to retrieve the author details for each track. We ended up extracting that logic to a different : Track.author.

Which of these are reasons why we extracted author-fetching logic to a different resolver in Lift-off II? (Hint: Keep reading if you need a refresher and come back to this question when you're ready!)

For the exact same reasons, we'll want to do the same here for our module details. Let's dig deeper into why.

⛓️ Resolver chains

We'll see this particular pattern often in . When we write a , we often have nested objects and . Take this query as an example:

query track(id: ‘c_0') {
  title
  author {
    name
  }
}

Remember that a is responsible for populating the data for a in your schema. It retrieves data from a . In our case, we have a resolver for our track that retrieves data from the REST API /track/:id endpoint.

Illustration showing a Query.track resolver retrieving data from the track/:id REST endpoint

It returns some of the properties our expects, such as title. It doesn't have the author name, but it does have the authorId, which is an ID we can use for another endpoint in our REST API, the author/:id endpoint.

We could tell this same to make the call to the author/:id endpoint, and then put together the results in the shape our expects.

However, this means that the is a bit overworked! If the doesn't ask for author information, the resolver would still do all of these unnecessary steps.

Illustration showing a Query.track resolver looking overworked as it retrieves data from two REST endpoints

So instead of putting all of that work on our poor Query.track , we can create another resolver function for Track.author. This is responsible for retrieving author information for a specific track. With that, we're forming a resolver chain!

Illustration showing a Query.track resolver linked with the Track.author resolver in a resolver chain

And remember the first parameter in a -- the parent? parent refers to the returned data of the preceding function in the chain! That's how we get access to the authorId from the track object, in our Track.author . Additionally, the Track.author will only be called when the asks for that !

Illustration showing the Query.track resolver passing the data to the Track.author resolver as a `parent` parameter

This pattern keeps each readable, easy to understand, and more resilient to future changes.

Resolver parameters
A resolver function populates the data for a 
 
 in your schema. The function has four parameters. The first, 
 
, contains the returned data of the previous function in the 
 
. The second, args, is an object that contains all the 
 
 provided to the field. We use the third parameter, 
 
, to access 
 
 such as a database or REST API. Finally, the fourth parameter, 
 
, contains informational properties about the operation state.

Drag items from this box to the blanks above

  • field

  • parent

  • info

  • context

  • arguments

  • data sources

  • type

  • value

  • resolver chain

  • variable

✍️ Adding a new resolver to the chain

Back to our specific problem, let's use this concept of chains to create a resolver for Track.modules.

We'll add this right below Track.author in the resolvers.js file:

modules: ({id}, _, {dataSources}) => {
  return dataSources.trackAPI.getTrackModules(id);
},

We'll destructure the first parameter to retrieve the id property from the parent, that's the id of the track. We don't need the args parameter, so that can be an underscore, and then destructure the third context parameter for the dataSources property.

Inside, we can return the results of calling our dataSources.trackAPI.getTrackModules method, passing in the id for the track.

And that's it for our ! With that, we've got our resolvers, our datasource and our schema all updated and ready to take on our new .

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