Gatsby Schema @link to MarkdownRemark slug from SitePage

gatsby-node.js

const path = require("path");
const { createFilePath } = require(`gatsby-source-filesystem`);

exports.createPages = async ({ graphql, actions }) => {
  const result = await graphql(
    `
      {
        allMarkdownRemark(sort: { fields: [frontmatter___date], order: DESC }) {
          edges {
            node {
              fields {
                slug
              }
            }
          }
        }
      }
    `
  );

  if (result.errors) {
    throw result.errors;
  }
  result.data.allMarkdownRemark.edges.forEach(({ node }) => {
    const component = path.resolve(`src/templates/post.js`);

    actions.createPage({
      path: node.fields.slug,
      component,
      context: {
        slug: node.fields.slug
      }
    });
  });
};

exports.onCreateNode = ({ node, getNode, actions }) => {
  if (node.internal.type === `MarkdownRemark`) {
    const slug = createFilePath({ node, getNode, basePath: `content` });

    actions.createNodeField({
      node,
      name: `slug`,
      value: slug
    });
  }
};

exports.createSchemaCustomization = ({ actions }) => {
  const typeDefs = `
    type SitePageFields {
      markdownRemark: MarkdownRemark @link(to: "fields.slug", from: "context.slug")
    }
    type SitePage implements Node {
      fields: SitePageFields
    }
  `;
  actions.createTypes(typeDefs);
};