gatsby-node.js

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

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

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

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

  return new Promise((resolve, reject) => {
    const component = path.resolve('./src/templates/card.js')
    resolve(
      graphql(
        `
          {
            allMarkdownRemark(limit: 1024) {
              edges {
                node {
                	frontmatter {
                	  author
                	}
                }
              }
            }
          }
        `
      ).then(result => {
        if (result.errors) {
          console.log(result.errors)
          reject(result.errors)
        }

        // Create blog posts pages.
        _.each(result.data.allMarkdownRemark.edges, edge => {
          createPage({
            path: edge.node.frontmatter.author,
            component,
            context: { author: edge.node.frontmatter.author }
          })
        })
      })
    )
  })
}