gatsby-node.js

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

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

  // Define a template for blog post
  const notebookView = path.resolve(`./src/templates/notebook.js`)
  const researchView = path.resolve(`./src/templates/research.js`)

  const generatePages = ({ pages, template }) => {
    pages.forEach((page, index) => {
      const previousPostId = index === 0 ? null : pages[index - 1].id
      const nextPostId = index === pages.length - 1 ? null : pages[index + 1].id
      createPage({
        path: page.fields.slug,
        component: template,
        context: {
          id: page.id,
          previousPostId,
          nextPostId,
        },
      })
    })
  }

  // Get all markdown blog posts sorted by date
  const result = await graphql(
    `
      {
        allFile(
          filter: {
            ext: { eq: ".md" }
            sourceInstanceName: { in: ["research", "notebook"] }
          }
          sort: { fields: childMarkdownRemark___frontmatter___date, order: ASC }
          limit: 1000
        ) {
          nodes {
            sourceInstanceName
            childMarkdownRemark {
              id
              fields {
                slug
              }
            }
          }
        }
      }
    `
  )

  if (result.errors) {
    reporter.panicOnBuild(
      `There was an error loading your blog posts`,
      result.errors
    )
    return
  }

  const nodes = [...result.data.allFile.nodes]

  const mdPages = nodes.map(page => {
    const instance = page.sourceInstanceName
    const pageSlug = page.childMarkdownRemark.fields.slug
    const path = `/${instance}${pageSlug}`
    return {
      id: page.childMarkdownRemark.id,
      instance,
      path,
      fields: {
        slug: path,
      },
    }
  })

  const researchPages = mdPages.filter(x => x.instance === "research")
  const notebookPages = mdPages.filter(x => x.instance === "notebook")

  // Create research pages
  if (researchPages.length > 0) {
    generatePages({ pages: researchPages, template: researchView })
  }
  if (notebookPages.length > 0) {
    generatePages({ pages: notebookPages, template: notebookView })
  }
}

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

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

    createNodeField({
      name: `slug`,
      node,
      value,
    })
  }
}

exports.createSchemaCustomization = ({ actions }) => {
  const { createTypes } = actions

  // Explicitly define the siteMetadata {} object
  // This way those will always be defined even if removed from gatsby-config.js

  // Also explicitly define the Markdown frontmatter
  // This way the "MarkdownRemark" queries will return `null` even when no
  // blog posts are stored inside "content/blog" instead of returning an error
  createTypes(`
    type SiteSiteMetadata {
      author: Author
      siteUrl: String
      social: Social
    }

    type Author {
      name: String
      summary: String
    }

    type Social {
      twitter: String
    }

    type MarkdownRemark implements Node {
      frontmatter: Frontmatter
      fields: Fields
    }

    type Frontmatter {
      title: String
      description: String
      date: Date @dateformat
    }

    type Fields {
      slug: String
    }
  `)
}