gatsby-node.js

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

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

  return graphql(`
    {
      allMarkdownRemark(limit: 1000) {
        edges {
          node {
            id
            fields {
              slug
            }
            frontmatter {
              templateKey
            }
          }
        }
      }
    }
  `).then(result => {
    if (result.errors) {
      result.errors.forEach(e => console.error(e.toString()));
      return Promise.reject(result.errors);
    }

    const posts = result.data.allMarkdownRemark.edges;

    posts
      .filter(
        ({
          node: {
            frontmatter: { templateKey }
          }
        }) => templateKey
      )
      .forEach(edge => {
        const {
          node: { id }
        } = edge;

        createPage({
          path: edge.node.fields.slug,
          component: path.resolve(`src/templates/${String(edge.node.frontmatter.templateKey)}.js`),
          context: {
            id
          }
        });
      });
  });
};

exports.onCreateNode = ({ node, actions, getNode }) => {
  const { createNodeField } = actions;

  if (node.internal.type === `MarkdownRemark`) {
    const value = createFilePath({ node, getNode });
    createNodeField({
      name: `slug`,
      node,
      value
    });
  }
};