gatsby-node.js

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

exports.onCreateNode = ({ node, getNode, actions }) => {
	const { createNodeField } = actions;
	if (node.internal.type === `MarkdownRemark`) {
		// Returns paths like (on Windows)
		// content\workshops\cms-created-test\index.md
		// content\links\link.md
		const relativePath = path.relative(`.`, node.fileAbsolutePath);
		const splitPath = relativePath.split(path.sep);

		// TODO
		// Modify so any markdown not matching these categories still has slug so no error occurs

		if (splitPath.length === 4 && splitPath[0] === `content` && splitPath[1] === `workshops`) {
			const slug = createFilePath({ node, getNode, basePath: `` });
			createNodeField({
				node,
				name: `slug`,
				value: `/workshops${slug}`,
			});

		// Create slug for files in src/pages/
		} else if (splitPath.length === 3 && splitPath[0] === `src` && splitPath[1] === `pages`) {
			const slug = createFilePath({ node, getNode, basePath: `src/pages/` });
			createNodeField({
				node,
				name: `slug`,
				value: slug,
			});
			createNodeField({
				node,
				name: `mainPage`,
				value: true,
			})
		}
	}
}