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
            }
          }
        }
      }
    }
  `).then((result) => {
    if (result.errors) {
      result.errors.forEach((error) => console.error(error.toString()))
      return Promise.reject(result.errors)
    }

    const posts = result.data.allMarkdownRemark.edges

    posts.forEach((edge) => {
      const { id } = edge.node
      createPage({
        path: edge.node.fields.slug,
        component: path.resolve("src/components/content-block.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,
    })
  }
}