gatsby-node.js

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

const chapterTemplate = path.resolve('src/templates/chapter.js')

function replacePath(pagePath) {
    return pagePath === `/` ? pagePath : pagePath.replace(/\/$/, ``)
}

async function onCreateNode({
    node,
    actions,
    getNode,
    loadNodeContent,
    createNodeId,
    createContentDigest,
}) {
    const { createNodeField, createNode, createParentChildLink } = actions
    if (node.internal.type === 'MarkdownRemark') {
        const parentDir = getNode(node.parent).relativeDirectory
        // Fix paths for Windows
        const dir = parentDir.replace(/\\/g, '/')
        const lang = dir.split('/')[0]
        const slug = createFilePath({ node, getNode, basePath: 'chapters', trailingSlash: false })
        createNodeField({ name: 'slug', node, value: slug })
        createNodeField({ name: 'lang', node, value: lang })
    } else if (node.extension === 'py') {
        // Load the contents of the Python file and make it available via GraphQL
        // https://www.gatsbyjs.org/docs/creating-a-transformer-plugin/
        const content = await loadNodeContent(node)
        const contentDigest = createContentDigest(content)
        const id = createNodeId(`${node.id}-code`)
        const internal = { type: 'Code', contentDigest }
        const codeNode = {
            id,
            parent: node.id,
            children: [],
            code: content,
            lang: node.relativeDirectory,
            name: node.name,
            internal,
        }
        createNode(codeNode)
        createParentChildLink({ parent: node, child: codeNode })
    }
}

exports.onCreateNode = onCreateNode

exports.createPages = ({ actions, graphql }) => {
    const { createPage } = actions
    return graphql(`
        {
            allMarkdownRemark {
                edges {
                    node {
                        frontmatter {
                            title
                            type
                        }
                        fields {
                            slug
                            lang
                        }
                    }
                }
            }
        }
    `).then(result => {
        if (result.errors) {
            return Promise.reject(result.errors)
        }
        const posts = result.data.allMarkdownRemark.edges.filter(
            ({ node }) => node.frontmatter.type == 'chapter'
        )
        posts.forEach(({ node }) => {
            createPage({
                path: replacePath(node.fields.slug),
                component: chapterTemplate,
                context: { slug: node.fields.slug, lang: node.fields.lang },
            })
        })
    })
}