gatsby-node.js

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

exports.onCreateNode = ({ node, getNode, actions }) => {
  const { createNodeField } = actions;
  if (node.internal.type === `MarkdownRemark`) {
    const { categories } = node.frontmatter;
    const slug = createFilePath({ node, getNode, basePath: `pages` });
    const [, date, title] = slug.match(
      /^\/([\d]{4}-[\d]{2}-[\d]{2})-{1}(.+)\/$/
    );
    const value = `/${slugify(
      categories.concat([date]).join("-"),
      "/"
    )}/${title}/`;
    createNodeField({ node, name: `slug`, value });
    createNodeField({ node, name: `date`, value: date });
  }
};

exports.createPages = ({ graphql, actions }) => {
  const { createPage } = actions;
  return new Promise((resolve, reject) => {
    graphql(`
      {
        allMarkdownRemark {
          edges {
            node {
              fields {
                slug
                date
              }
              frontmatter {
                categories
              }
            }
          }
        }
      }
    `).then(result => {
      result.data.allMarkdownRemark.edges.map(({ node }) => {
        createPage({
          path: node.fields.slug,
          component: path.resolve(`./src/templates/blog-post.js`),
          context: {
            // Data passed to context is available in page queries as GraphQL variables.
            slug: node.fields.slug,
            date: node.fields.date
          }
        });
      });
      resolve();
    });
  });
};