single rss.xml from 2 sources (MDX & Sanity.io)

gatsby-config.js

// Load variables from `.env` as soon as possible
require('dotenv').config({
  path: `.env.${process.env.NODE_ENV || 'development'}`
})

const clientConfig = require('./client-config')
const isProd = process.env.NODE_ENV === 'production'

// Portable Text Serialization
const PortableText = require("@sanity/block-content-to-html")
const imageUrlBuilder = require("@sanity/image-url")
const h = PortableText.h
const imageUrlFor = source => imageUrlBuilder(clientConfig.sanity).image(source)

// Helper functions for Portable Text
const { isFuture } = require("date-fns")
function filterOutDocsPublishedInTheFuture({ publishedAt }) {
  return !isFuture(publishedAt);
}
function getBlogUrl(slug) {
  return `/journal/${slug.current || slug}/`
}

module.exports = {
  siteMetadata: {
    title: "Ajmal Afif",
    siteUrl: "https://afif.dev",
    description: "🌐 of Ajmal Afif",
  },
  plugins: [
    'gatsby-plugin-react-helmet',
    {
      resolve: 'gatsby-source-sanity',
      options: {
        ...clientConfig.sanity,
        token: process.env.SANITY_READ_TOKEN,
        watchMode: !isProd,
        overlayDrafts: !isProd
      }
    },
    {
      resolve: "gatsby-plugin-sanity-image",
      options: {
        ...clientConfig.sanity,
        defaultImageConfig: {
          quality: 75,
          fit: "max",
          auto: "format",
        },
      },
    },
    {
      resolve: `gatsby-source-filesystem`,
      options: {
        name: `pages`,
        path: `${__dirname}/src/content`,
      },
    },
    {
      resolve: `gatsby-source-filesystem`,
      options: {
        name: `work`,
        path: `${__dirname}/src/content/work`,
      },
    },
    {
      resolve: `gatsby-source-filesystem`,
      options: {
        path: `${__dirname}/src/pages`,
      },
    },
    'gatsby-plugin-mdx',
    'gatsby-plugin-emotion',
    'gatsby-plugin-theme-ui',
    {
      resolve: `gatsby-plugin-feed`,
      options: {
        query: `
          {
            site {
              siteMetadata {
                title
                description
                siteUrl
                site_url: siteUrl
              }
            }
          }
        `,
        feeds: [
          {
            serialize: ({ query: { site, allSanityPost = [], allMdx } }) => {
              const records = [];
              allSanityPost.edges
                .filter(({ node }) => filterOutDocsPublishedInTheFuture(node))
                .filter(({ node }) => node.slug)
                .forEach(({ node }) => {
                  const { title, publishedAt, slug, _rawBody, _rawExcerpt } = node;
                  const url = site.siteMetadata.siteUrl + getBlogUrl(slug.current);
                  records.push({
                    title: title,
                    date: publishedAt,
                    url,
                    guid: url,
                    description: PortableText({
                      blocks: _rawExcerpt,
                      serializers: {
                        types: {
                          code: ({ node }) =>
                            h("pre", h("code", { lang: node.language }.node.code))
                        }
                      }
                    }),
                    custom_elements: [
                      {
                        "content:encoded": PortableText({
                          blocks: _rawBody,
                          serializers: {
                            types: {
                              code: ({ node }) =>
                                h("pre", h("code", { lang: node.language }, node.code)),
                              mainImage: ({ node }) =>
                                h("img", {
                                  src: imageUrlFor(node.asset).url()
                                }),
                              authorReference: ({ node }) => h("p", "Author: " + node.author.name),
                            }
                          }
                        })
                      }
                    ]
                  });
                });

              allMdx.edges.forEach(edge => {
                records.push(Object.assign({}, edge.node.frontmatter, {
                  description: edge.node.excerpt,
                  date: edge.node.frontmatter.date,
                  url: site.siteMetadata.siteUrl + edge.node.fields.slug,
                  guid: site.siteMetadata.siteUrl + edge.node.fields.slug,
                  custom_elements: [{ 'content:encoded': edge.node.html }],
                }));
              });

              return records;
            },
            query: `
            {
              allSanityPost(sort: { fields: publishedAt, order: DESC }) {
                edges {
                  node {
                    _rawExcerpt
                    _rawBody(resolveReferences: {maxDepth: 10})
                    title
                    publishedAt
                    slug {
                      current
                    }
                  }
                }
              }
              allMdx(
                limit: 1000,
                sort: { order: DESC, fields: [frontmatter___date] },
                filter: { frontmatter: { template: { regex: "/work-post/" } }}
              ) {
                edges {
                  node {
                    fields {
                      slug
                    }
                    frontmatter {
                      title
                      date
                    }
                    html
                  }
                }
              }
            }
            `,
            output: "/rss.xml",
            title: "Ajmal Afif — RSS Feed",
          }
        ]
      }
    },
  ]
}

credits to @dylanjha for figuring it out!