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 blogPost = path.resolve(`./src/templates/blog-post.js`);

  // Get all markdown blog posts sorted by date
  const result = await graphql(
    `
      {
        allMarkdownRemark(
          filter: { fields: { contentType: { eq: "blog" } } }
          sort: { fields: [frontmatter___date], order: DESC }
          limit: 1000
        ) {
          nodes {
            id
            fields {
              slug
            }
            frontmatter {
              title
            }
          }
        }
      }
    `
  );

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

  const posts = result.data.allMarkdownRemark.nodes;

  // Create blog posts pages
  // But only if there's at least one markdown file found at "content/blog" (defined in gatsby-config.js)
  // `context` is available in the template as a prop and as a variable in GraphQL

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

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

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

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

    let slugPrefix = "";

    if (type !== "page") {
      slugPrefix = `/${type}`;
    }

    if (type === "blog") {
      slugPrefix = `${slugPrefix}/post`;
    }

    createNodeField({
      name: `slug`,
      node,
      value: `${slugPrefix}${value}`,
    });
    createNodeField({
      name: `contentType`,
      node,
      value: type,
    });
  }
};

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
      spotify: String
    }

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

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

    type Fields {
      slug: String
      contentType: String
    }
  `);
};