gatsby-node.js

const _ = require('lodash')
const Promise = require('bluebird')
const path = require('path')
const { createFilePath } = require('gatsby-source-filesystem')

exports.createPages = ({ graphql, boundActionCreators }) => {
  const { createPage } = boundActionCreators

  return new Promise((resolve, reject) => {
    const lesson = path.resolve('./src/templates/lesson.js')
    resolve(
      graphql(
        `
          {
            allMarkdownRemark(
              limit: 1000
            ) {
              edges {
                node {
                  fields {
                    slug
                  }
                  frontmatter {
                    title
                    week
                  }
                }
              }
            }
          }
        `
      ).then(result => {
        if (result.errors) {
          console.log(result.errors)
          reject(result.errors)
        }

        // Create lessons pages.
        const lessons = result.data.allMarkdownRemark.edges

        _.each(lessons, (post, index) => {
          const previous =
            index === lessons.length - 1 ? false : lessons[index + 1].node
          const next = index === 0 ? false : lessons[index - 1].node

          createPage({
            path: post.node.fields.slug,
            component: lesson,
            context: {
              slug: post.node.fields.slug,
            },
          })
        })
      })
    )
  })
}

exports.onCreateNode = ({ node, boundActionCreators, getNode }) => {
  const { createNodeField } = boundActionCreators

  if (node.internal.type === `MarkdownRemark`) {
    const value = createFilePath({ node, getNode })
    createNodeField({
      name: `slug`,
      node,
      value,
    })
  }
}