Let's assume you want to extend a third party GraphQL API with a custom field. You have no control over the API and can't change it. This guide shows you how to extend the Schema and add custom logic to the Gateway to resolve the custom field.
Extend the Schema
First, we need to introspect the API and extend the Schema with the custom field.
In this case, we extend the
Capsule type with a custom field called
specification. The specification field will be a computed field combining
Note that we use the
extend keyword to extend the type. If you're familiar with API Namespacing, you can see that we're not using a namespace when extending the type. That's because the Schema Extension is applied before namespacing.
Also, worth noting is that the
specification is of type
String, which means that it's nullable. This is important because the WunderGraph Engine will not be able to resolve the field from the third party API. This means, the field will come back as
null from the Engine Resolver.
Resolve the custom field
You're now able to extend the Schema and understand that the field will come back as
null. Let's add some custom logic to resolve the field.
Before we can add custom logic, we need to create an Operation that uses the extended Schema.
As you can see, we're querying the
specification on the
Capsule type alongside other fields from the third party API. Now let's add a hook to resolve the field.
We're adding a
mutatingPostResolve hook to the
Capsule query. This hook is called after the Engine Resolver has resolved the other fields from the third party API.
The final response will look like this:
You've learned how to extend third party GraphQL APIs with WunderGraph. In the example, we've simply added a custom field which is computed from other fields. However, you can add any custom logic to the
mutatingPostResolve hook, e.g. you could call another API to fetch additional data and add it to the response.