gatsby-node.js

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

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

  const blogPostTemplate = path.resolve(`./src/templates/blog-post.js`)
  const tagTemplate = path.resolve(`./src/templates/tag.js`)

  return graphql(
    `
      {
        allMarkdownRemark(
          sort: { fields: [frontmatter___date], order: DESC }
          limit: 1000
        ) {
          edges {
            node {
              fields {
                slug
              }
              frontmatter {
                title
              }
            }
          }
        }
        tagsGroup: allMarkdownRemark(limit: 2000) {
          group(field: frontmatter___tags) {
            fieldValue
          }
        }  
      }
    `
  ).then(result => {
    if (result.errors) {
      throw result.errors
    }

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

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

      createPage({
        path: `/blog${post.node.fields.slug}`,
        component: blogPostTemplate,
        context: {
          slug: post.node.fields.slug,
          previous,
          next,
        },
      })
    })

    // Extract tag data from query
    const tags = result.data.tagsGroup.group
    // Make tag pages
    tags.forEach(tag => {
      createPage({
        path: `/tags/${tag.fieldValue.replace(/\s/,"-").toLowerCase()}/`,
        component: tagTemplate,
        context: {
          tag: tag.fieldValue,
        },
      })
    })


    return null
  })
}

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

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