gatsby-node.js

const path = require("path");

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

  return graphql(`
    {
      allMarkdownRemark(
        sort: { order: DESC, fields: [frontmatter___date] }
        limit: 1000
      ) {
        edges {
          node {
            excerpt(pruneLength: 400)
            html
            id
            frontmatter {
              templateKey
              path
              date
              title
              description
            }
          }
        }
      }
    }
  `).then(result => {
    if (result.errors) {
      result.errors.forEach(e => console.error(e.toString()));
      return Promise.reject(result.errors);
    }

    return result.data.allMarkdownRemark.edges.forEach(({ node }) => {
      const pagePath = node.frontmatter.path;
      const templateKey = node.frontmatter.templateKey || 'page'
      createPage({
        path: pagePath,
        component: path.resolve(
          `src/templates/${String(templateKey)}.js`
        ),
        // additional data can be passed via context
        context: {
          path: pagePath
        }
      });
    });
  });
};