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') {
    const slug = createFilePath({ node, getNode, basePath: 'pages' })
    createNodeField({
      node,
      name: 'slug',
      value: slug
    })
  }
}

exports.createPages = ({ graphql, actions }) => {
  const { createPage } = actions
  return graphql(`
    {
      allMarkdownRemark(
      sort: {fields: [frontmatter___date], order: DESC},
      ${
        process.env.NODE_ENV === 'production'
          ? 'filter: {frontmatter: {draft: {ne: true}}}'
          : ''
      }
      ) {
        edges {
          node {
            fields {
              slug
            }
          }
        }
      }
    }
  `).then(result => {
    result.data.allMarkdownRemark.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
        }
      })
    })
  })
}