gatsby-node.js

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

exports.onCreateNode = ({ node, getNode, actions }) => {
  const { createNodeField } = actions;
  if (
    node.internal.type === "MarkdownRemark" &&
    node.frontmatter.title !== ""
  ) {
    const slug = createFilePath({
      node,
      getNode,
    });
    createNodeField({
      node,
      name: "slug",
      value: `${slug}`,
    });
  }
};

exports.createPages = ({ graphql, actions }) => {
  const { createPage } = actions;
  const pagePaths = {
    blog: "./src/blog/PostPage.js",
    garden: "./src/digital-garden/PostPage.js"
  };
  return new Promise((resolve, reject) => {
    graphql(`
    {
      allMarkdownRemark(
        filter: {
          fileAbsolutePath: { ne: null }
          frontmatter: { date: { ne: null } }
        }
      ) {
        edges {
          node {
            fields {
              slug
            }
            frontmatter {
              type
            }
          }
        }
      }
    }
    `).then(result => {
      result.data.allMarkdownRemark.edges.forEach(({ node }) => {
        try {
          if (node && node.frontmatter && node.frontmatter.type) {
            createPage({
              path: node.fields.slug,
              component: path.resolve(pagePaths[node.frontmatter.type]),
              context: {
                slug: node.fields.slug,
              },
            });
          }
        } catch (error) {
          console.log(error);
        }
      });
      resolve();
    });
  });
};