Add section on directives to reference documentation

[cforce]

Provide docs and example to serve content for a directive schema query, e.g by using RuntimeWiringConfigurer
https://graphql.org/learn/queries/#directives

[hantsy]

@cforce Check my example: https://github.com/hantsy/spring-graphql-sample/blob/master/spring-graphql/src/main/java/com/example/demo/gql/PostsRuntimeWiring.java#L42

[rstoyanchev]

Under RuntimeWiringConfigurer in the Spring GraphQL docs, it is mentioned that this is the hook for directives, i.e. by getting access to the RuntimeWiring.Builder, and that you can declare a RuntimeWiringConfigurer as a bean. The rest should be evident from the options on RuntimeWiring.Builder API:

@Bean
public RuntimeWiringConfigurer directivesConfigurer() {
	return builder -> {
		builder.directiveWiring(...);
	};
}

Perhaps, if you're not familiar with directives in GraphQL Java in the first place, then I can see how the docs could seem a bit lacking, but really that is for GraphQL Java to document, see the GraphQL Java page on SDL directives.

Perhaps, we could expand the Spring GraphQL docs a bit to link to the GraphQL Java documentation on directives, and encourage reading that first. After that, the rest should be easier maybe, or if not please suggest a way to improve?

[cforce]

Yes, sounds like a good idea to link and add some / this basic example to the examples in repo as well.

[rstoyanchev]

Okay, let me use this issue then for that as a doc improvement.

[marceloverdijk]

Personally I would like to understand how to write validation / @constraint directive that in case validation fails a GraphQL error is send to the client.

This as alternative to having @Valid annotations in controllers and instead define them in the schema.