GatsbyJS - How to Add Slug Field

Add slug field in gatsby-node.js:

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

exports.onCreateNode = ({ node, getNode, actions }) => {
  const { createNodeField } = actions;
  if (node.internal.type === `MarkdownRemark`) {
    // ...

    const slug = createFilePath({ node, getNode, basePath: `pages` });
    createNodeField({
      node,
      name: `slug`,
      value: slug,
    });

    // ...
  }
};

slug will be the file path after the pages, e.g. if you have a page project_folder/src/pages/en/my_first_page.md, the slug would be /en/my_first_page/.

Then the slug can be queried by

query {
  allMarkdownRemark {
    edges {
      node {
        fields {
          slug
        }
      }
    }
  }
}