Use parent's resolver using custom field extension. FTW!!

gatsby-node.js

exports.createSchemaCustomization = ({ actions }) => {
  
  const { createFieldExtension, createTypes } = actions;
  
  createFieldExtension({
    name: `parent`,
    description: `Proxy resolver from a parent's field.`,
    args: {
      from: `String!`
    },
    extend(options, fieldConfig) {
      return {
        async resolve(source, args, context, info) {
          const fieldName = options.from || info.from;
          const parentNode = context.nodeModel.getNodeById({
            id: source.parent
          });
          const type = info.schema.getType(parentNode.internal.type);
          const resolver = type.getFields()[fieldName].resolve;
          const result = await resolver(parentNode, args, context, {
            fieldName
          });
          return result;
        }
      };
    }
  });

  createTypes(`type MdxCustom implements Node & Custom {
    id: ID!
    slug: String @proxy(from:"frontmatter.slug")
    body: String @parent(from:"body")
  }`);

}