gatsby-node.js

const path = require("path");

exports.createPages = ({ boundActionCreators, graphql }) => {
  const { createPage } = boundActionCreators;

  const lessonTemplate = path.resolve(`src/templates/lessonTemplate.js`);

  return graphql(`
    {
      allMarkdownRemark(
        sort: { order: DESC, fields: [frontmatter___order] }
        limit: 1000
      ) {
        edges {
          node {
            excerpt(pruneLength: 250)
            html
            id
            frontmatter {
              order
              path
              title
            }
          }
        }
      }
    }
  `).then(result => {
      if (result.errors) {
        return Promise.reject(result.errors);
      }

      result.data.allMarkdownRemark.edges.forEach(({ node }) => {
        createPage({
          path: node.frontmatter.path,
          component: lessonTemplate
        });
      });
    });
};