I have a node project using GraphQL(v16.7.0) and the @graphql-tools/utils library, attempting to setup a nonEmptyArray custom schema directive to throw an error whenever a mutation is called with an empty array for an input. It would be used like this:

input ExampleInput {
  ids: [String!]! @nonEmptyArray
}

The code I have so far is:

const getNonEmptyArrayDirective = (schema: GraphQLSchema): SchemaMapper => ({
  [MapperKind.INPUT_OBJECT_FIELD]: (
    fieldConfig: GraphQLInputFieldConfig,
    fieldName: string,
    typeName: string,
  ) => {
    const directive = getDirective(schema, fieldConfig, 'nonEmptyArray')?.[0];
    if (directive) {
      fieldConfig.extensions = {
        ...fieldConfig.extensions,
        validateInput: inputValue => {
          console.log({inputValue})
          if (Array.isArray(inputValue)) {
            if (inputValue.length === 0) {
              throw new EmptyArrayError(fieldName);
            }
          }

          return inputValue;
        },
      };
    }
    return fieldConfig;
  },
});

export default function applySchemaDirectives(
  schema: GraphQLSchema,
): GraphQLSchema {
  schema = mapSchema(schema, getNonEmptyArrayDirective(schema));

  return schema;
}

and declaring the directive:

directive @nonEmptyArray on FIELD_DEFINITION | INPUT_FIELD_DEFINITION

So far I can’t even get the console.log({inputValue}) to execute. I’ve experimented with adding other MapperKind values such as MapperKind.SCALAR_TYPES and those do log properly, implying it’s not a problem with the general setup of the custom directive. My impression is that the issue has something to do with the way GraphQL specifically handles list types.