gatsby-node.js

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

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

  return graphql(
    `
      {
        allMarkdownRemark(
          sort: { fields: [frontmatter___date], order: DESC }
          limit: 1000
        ) {
          edges {
            node {
              fields {
                slug
              }
              frontmatter {
                title
                date(formatString: "YYYY.MM.DD")
                emoji
                category
                difficulty
              }
            }
          }
        }
      }
    `
  ).then((result) => {
    if (result.errors) {
      throw result.errors;
    }

    const posts = result.data.allMarkdownRemark.edges;

    // Create category posts pages
    // ref: https://www.gatsbyjs.org/docs/adding-tags-and-categories-to-blog-posts/
    let categories = new Set();
    posts.forEach((post) => {
      if (post.node.frontmatter.category) {
        for (let eachCategory of post.node.frontmatter.category)
          categories.add(eachCategory);
      }
    });

    categories.forEach((category) => {
      createPage({
        path: `/category/${category}/`,
        component: path.resolve("src/templates/categories.js"),
        context: {
          category,
        },
      });
    });

    // get related Posts(retrive maximum 5 posts for each category)
    let allRelatedPosts = {};
    categories.forEach((category) => {
      let categoryPosts = posts.filter((post) => {
        return post.node.frontmatter.category.includes(category);
      });
      allRelatedPosts[category] = categoryPosts
        ? categoryPosts.slice(0, 5)
        : [];
    });

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

      // setup related posts
      // get the posts that has same categories.

      let relatedPosts = _.filter(posts, (eachPost) => {
        if (
          !(eachPost.node.fields.slug === post.node.fields.slug) &&
          _.intersection(
            post.node.frontmatter.category,
            eachPost.node.frontmatter.category
          ).length
        ) {
          return true;
        }
      });

      createPage({
        path: post.node.fields.slug,
        component: path.resolve(`./src/templates/post.js`),
        context: {
          slug: post.node.fields.slug,
          relatedPosts,
        },
      });
    });
  });
};

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

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