gatsby-node.js

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

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

  return graphql(`
    {
      allMarkdownRemark(limit: 1000) {
        edges {
          node {
            id
            fields {
              slug
            }
            frontmatter {
              templateKey
            }
          }
        }
      }
    }
  `).then(result => {
      if (result.errors) {
        result.errors.forEach(e => console.error(e.toString()))
        return Promise.reject(result.errors)
      }

      const posts = result.data.allMarkdownRemark.edges

      posts.forEach(edge => {
        const id = edge.node.id
        createPage({
          path: edge.node.fields.slug,
          tags: edge.node.frontmatter.tags,
          component: path.resolve(
            `src/templates/${String(edge.node.frontmatter.templateKey)}.js`
          ),
          // additional data can be passed via context
          context: {
            id,
          },
        })
      })
    })
}

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

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