Unlocking the Power of GraphQL: A Journey with Union Types
As I dove into building a Nuxt.js and GraphQL application, I stumbled upon an error that left me perplexed: “You are using the simple (heuristic) fragment matcher, but your queries contain union or interface types. Apollo Client will not be able to accurately map fragments.” The culprit? Union types. But what exactly are they, and how do we harness their power?
The Mystery of Union Types
In GraphQL, union types are abstract types that indicate a field can return more than one object type. Think of them as a container that holds multiple types, but doesn’t define specific fields itself. When I created a Dynamic Zone using Strapi’s GraphQL API, each component in it acted as a field, and I had three types in my pageZone object: Image, Quote, and Video. This changes how we make queries, and it’s not as simple as querying union types like regular objects.
The Apollo Client Conundrum
The default heuristic fragment matcher in Apollo Client won’t work accurately with union types. That’s where the IntrospectionFragmentMatcher comes in. This powerful tool looks at your schema and generates all possible types, so when you send a query containing Union types, Apollo Client can confidently return data.
Setting Up GraphQL Code Generator
To implement the IntrospectionFragmentMatcher, we need to use GraphQL Code Generator. This tool helps us avoid making heuristic queries. After installing it, we create a codegen.yml file to configure our code generator. This YAML file acts as a config store for our code generator, defining our schema endpoint and output directory for our generated introspection file.
Rendering Query Results
Now that we’ve set up Apollo Client, it’s time to render our query results. In a Nuxt.js application, we use the v-for directive to loop over the elements in our query result. We create zones to display data that matches their type, using the __typename to compare with the type of the Image component.
The Logic Behind Displaying Data
The core logic behind displaying this data is:
- Looping through the query result
- Looping through the Union type
- Creating “zones” by conditionally displaying components
This approach can be applied to any frontend, with slight variations in Apollo config directories.
Embracing the Flexibility of Union Types
GraphQL is an incredibly powerful tool that enables us to do a lot. By understanding union types and solving the problem of heuristic queries with Apollo Client, we can unlock the full potential of GraphQL. Have you ever used Union types in a project? Share your experiences and favorite types on Twitter!