gatsby-node.js

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

const { postsPerPage } = require('./lombardi.config')

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

  const result = await graphql(
    `
      {
        allMarkdownRemark(
          sort: { fields: [frontmatter___date], order: DESC }
          filter: { frontmatter: { templateKey: { eq: "BlogPost" } } }
          limit: 1000
        ) {
          edges {
            node {
              id
              fields {
                slug
              }
              frontmatter {
                title
                url
                templateKey
              }
            }
          }
        }
      }
    `
  )

  if (result.errors) {
    throw result.errors
  }

  const posts = result.data.allMarkdownRemark.edges

  // Create blog page (blog listing)
  const numPages = Math.ceil(posts.length / postsPerPage)
  Array.from({ length: numPages }).forEach((_, i) => {
    createPage({
      path: i === 0 ? `/blog/` : `/blog/${i + 1}`,
      component: path.resolve('./src/templates/BlogPage.jsx'),
      context: {
        limit: postsPerPage,
        skip: i * postsPerPage,
        numPages,
        currentPage: i + 1,
      },
    })
  })

  // Create blog posts pages (individual post page)
  posts.forEach((post, index) => {
    const previous = index === posts.length - 1 ? null : posts[index + 1].node
    const next = index === 0 ? null : posts[index - 1].node

    createPage({
      path: `/blog/${post.node.frontmatter.url}`,
      component: path.resolve(`src/templates/${String(post.node.frontmatter.templateKey)}.jsx`),
      context: {
        slug: post.node.fields.slug,
        previous,
        next,
      },
    })
  })
}

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

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