gatsby-node.js

const fs = require('fs');
const path = require('path');
const _ = require('lodash');
const { createFilePath } = require('gatsby-source-filesystem');
const { siteMetadata } = require('./gatsby-config');

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

  const blogPost = path.resolve('./src/templates/blog-post.tsx');
  const result = await graphql(`
    {
      allMarkdownRemark(
        filter: { fields: { collection: { eq: "blog" } } }
        sort: { fields: frontmatter___date, order: DESC }
      ) {
        edges {
          node {
            fields {
              slug
            }
            frontmatter {
              title
              date
              commentsUrl
            }
          }
        }
      }
    }
  `);

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

  // Create blog posts pages.
  const posts = result.data.allMarkdownRemark.edges;

  posts.forEach(({ node: post }, index) => {
    const previous = index === posts.length - 1 ? null : posts[index + 1].node;
    const next = index === 0 ? null : posts[index - 1].node;

    createPage({
      path: post.fields.slug,
      component: blogPost,
      context: {
        slug: post.fields.slug,
        previous,
        next
      }
    });
  });
};

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

  if (node.internal.type === 'MarkdownRemark') {
    // Get the parent node
    const parent = getNode(_.get(node, 'parent'));
    // Create a field on this node for the "collection" of the parent
    // NOTE: This is necessary so we can filter `allMarkdownRemark` by
    // `collection` otherwise there is no way to filter for only markdown
    // documents of type `post`.
    createNodeField({
      node,
      name: 'collection',
      value: _.get(parent, 'sourceInstanceName')
    });

    // Get slug
    const value = createFilePath({ node, getNode }).replace(/^\/[0-9\-]*/, '/');
    createNodeField({
      name: 'slug',
      node,
      value
    });
  }
};

// TODO dont use generated public folder, create file in static folder static/api/posts.json in createPages hook
// Create json to use on https://dvc.org/community
exports.onPostBuild = async function({ graphql }) {
  const result = await graphql(`
    {
      allMarkdownRemark(
        filter: { fields: { collection: { eq: "blog" } } }
        sort: { fields: frontmatter___date, order: DESC }
        limit: 3
      ) {
        edges {
          node {
            fields {
              slug
            }
            frontmatter {
              title
              date
              commentsUrl
            }
          }
        }
      }
    }
  `);

  if (result.errors) {
    throw new Error(result.errors);
  }

  const posts = result.data.allMarkdownRemark.edges.map(
    ({
      node: {
        fields: { slug },
        frontmatter: { title, date, commentsUrl }
      }
    }) => {
      const url = `${siteMetadata.siteUrl}/${slug}`;

      return {
        url,
        title,
        date,
        commentsUrl
      };
    }
  );

  const dir = path.join(__dirname, '/public/api');
  const filepath = path.join(dir, 'posts.json');

  // Write json file to the public dir,
  // it will be used community page later
  if (!fs.existsSync(dir)) fs.mkdirSync(dir);
  fs.writeFileSync(filepath, JSON.stringify({ posts }));
};

/*
  // Create /api/posts.json
  const data = posts.slice(0, 3).map(
    ({
      node: {
        fields: { slug },
        frontmatter: { title, date, commentsUrl }
      }
    }) => {
      const url = `${siteMetadata.siteUrl}${slug}`;
      return {
        url,
        title,
        date,
        commentsUrl
      };
    }
  );
  const filepath = path.resolve(__dirname, 'static', 'api', 'posts.json');
  fs.writeFileSync(filepath, JSON.stringify({ posts: data }));
*/