gatsby-node.js

const crypto = require('crypto');
const get = require('lodash.get');

const setNullToEmpty = data =>
  Object.keys(data).reduce((merged, key) => {
    if (key !== 'null') {
      merged[key] = data[key] === null ? '' : data[key];
    }
    return merged;
  }, {});

/*
 * This is a utility plugin that will enable
 * linking Markdown metadata for further linking
 * It's currently used to link Markdown.frontmatter.author
 * to an actual person, and "joins" on id
 * We could/should open source this and release it
 */
exports.onCreateNode = function onCreateNode(
  { node, boundActionCreators, loadNodeContent, getNode },
  pluginOptions
) {
  if (node.internal.type !== 'MarkdownRemark') {
    return;
  }

  const { createNode, createParentChildLink } = boundActionCreators;

  const { sourceInstanceName: type } = getNode(node.parent);
  if (type === 'person') {
    const { frontmatter } = node;
    const data = setNullToEmpty(frontmatter);
    const contentDigest = crypto
      .createHash('md5')
      .update(JSON.stringify(data))
      .digest('hex');

    const id = get(node, pluginOptions.id);

    const personNode = Object.assign(data, {
      id,
      children: [],
      parent: node.id,
      internal: {
        contentDigest,
        type: pluginOptions.type,
      },
    });

    createNode(personNode);
    createParentChildLink({ parent: node, child: personNode });
  }
};