How to add a directive for arguments in graphql?

@herminia_bruen 

To add a directive for arguments in GraphQL, you need to first define the directive in your schema. Directives are declared using the directive keyword and specifying the name, arguments, and locations where it can be applied.

For example, let's create a directive called capitalize that can be used to transform a string argument to uppercase. Here's how you can define the directive in the schema:

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directive @capitalize on ARGUMENT_DEFINITION

input MyQueryInput {
  name: String @capitalize
}

type Query {
  myQuery(input: MyQueryInput): String
}

In this example, we've defined a directive called @capitalize that can be applied to arguments. We then used this directive on the name argument of the MyQueryInput input type.

To implement the logic for the directive, you need to define a custom directive resolver in your GraphQL server. This resolver will be responsible for processing the arguments and applying the directive logic.

For example, in a GraphQL server implemented using Apollo Server, you can define a directive resolver like this:

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const { SchemaDirectiveVisitor } = require('graphql-tools');

class CapitalizeDirective extends SchemaDirectiveVisitor {
  visitInputFieldDefinition(field) {
    const { resolve = defaultFieldResolver } = field;
    const { directiveArgs } = this.args;
    
    field.resolve = async function (source, args, context, info) {
      const value = await resolve.call(this, source, args, context, info);
      
      if (directiveArgs && directiveArgs.value) {
        // Apply the directive logic
        return value.toUpperCase();
      }
      
      return value;
    };
  }
}

module.exports = {
  CapitalizeDirective
};

You'll then need to apply the directive to any fields in your resolvers that you want to transform using the @capitalize directive. When querying the schema, you can use the directive like this:

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query {
  myQuery(input: { name: "john" }) {
    name
  }
}

This will transform the name argument to uppercase before executing the resolver logic.