gatsby-node.js

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

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

  const categoryTemplate = path.resolve('src/templates/CategoryTemplate.tsx')
  const result = await graphql(`
    {
      allMdx {
        group(field: { frontmatter: { category: SELECT } }) {
          fieldValue
        }
      }
    }
  `)

  if (result.errors) {
    reporter.panicOnBuild(`Error while running GraphQL query.`)
    return
  }

  const categories = result.data.allMdx.group

  categories.forEach((category) => {
    createPage({
      path: `/post/category/${category.fieldValue}`,
      component: categoryTemplate,
      context: {
        category: category.fieldValue,
      },
    })
  })
}

exports.onCreateNode = ({ node, actions, getNode }) => {
  const { createNodeField } = actions
  if (node.internal.type === `Mdx`) {
    const value = createFilePath({ node, getNode })
    const { sourceInstanceName } = getNode(node.parent)
    createNodeField({
      name: `slug`,
      node,
      value,
    })
    createNodeField({
      node,
      name: 'source',
      value: sourceInstanceName,
    })
  }
}