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 === `Mdx`) {
      const fileNode = getNode(node.parent)
      console.log(`\n`, fileNode.relativePath)
      console.log(createFilePath({ node, getNode, basePath: `pages` }))
      const slug = createFilePath({ node, getNode, basePath: `pages` })
      createNodeField({
        node,
        name: `slug`,
        value: slug,
      })
    }
  }

  exports.createPages = async ({ graphql, actions }) => {
    // **Note:** The graphql function call returns a Promise
    // see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise for more info
    const {createPage} = actions;
    const result = await graphql(`
      query {
        allMdx {
          edges {
            node {
              fields {
                slug
              }
            }
          }
        }
      }
    `)
    console.log('createPages:', JSON.stringify(result, null, 4))

    result.data.allMdx.edges.forEach(({ 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, // ex. "sweet-pandas-eating-sweets/"
          },
        })
      })
}