Is there a workaround for customising schema from a Gatsby Theme?

[JohJonker]

Summary

I'm trying to specify schema using exports.createSchemaCustomization = ({ actions }, siteOptions) in my Gatsby Theme, but it's being ignored by the site that uses the theme. This is a known limitation which has seen some discussion (#15544, #16529).

My question: As of now, is there a workaround to do this?

I know we could move this to the site's gatsby-node.js, but that's unideal — as it creates unnecessary code duplication across sites, when it could just live in the theme — and I'm also having issues (#24427) accessing the config in the site.

Relevant information

My Gatsby theme allows my sites to specify Markdown content types with frontmatter fields, like so:

gatsby-config.js:

module.exports = {
  plugins: [
    {
      resolve: `MY_THEME_SAMPLE`,
      options: {
        contentTypes: [
          {
            id: 'news',
            name: 'News',
            urlBase: '/news/',
            singleTemplate: 'NewsArticle',
            archiveTemplate: 'News',
            frontmatterFields: [
              { name: 'thumbnail', type: 'File!' },
              { name: 'title', type: 'String!' },
              { name: 'date', type: 'Date!' },
              { name: 'summary', type: 'String!' },
              {
                name: 'people',
                type:
                  '[MarkdownRemark]! @link(by: "fields.slug", from: "people")',
              },
              { name: 'topics', type: '[String]!' },
            ],
          },
        ]
      }
    }
  ]
}

Ideally, I would've liked then liked to use this info to generate schema customisations in the theme's gatsby-node.js, like below. This code currently runs, but it's being ignored, i.e. the schema customisations aren't being applied. In the build output on the site using the theme, I get:

warn Plugin 'MY_THEME_SAMPLE' tried to define the GraphQL type 'MarkdownRemark', which has already been defined by the plugin

// Explicitly specify GraphQL schema for better performance and reliability
exports.createSchemaCustomization = ({ actions }, siteOptions) => {
  const { createTypes } = actions;

  const { contentTypes } = createConfig(siteOptions);
  const contentTypeDefinitions = {};
  for (let i = 0; i < contentTypes.length; i += 1) {
    for (let j = 0; j < contentTypes[i].frontmatterFields.length; j += 1) {
      if (
        contentTypeDefinitions[contentTypes[i].frontmatterFields[j].name] ===
        undefined
      ) {
        // Checks for duplicate field declarations; only the first will be declared
        contentTypeDefinitions[contentTypes[i].frontmatterFields[j].name] =
          contentTypes[i].frontmatterFields[j].type;
      }
    }
  }
  const finalDefinitions = `
  type MarkdownRemark implements Node {
    fields: Fields
    frontmatter: Frontmatter    
  }
  type Fields {
    slug: String
    collection: String
  }
  type Frontmatter {
      ${Object.keys(contentTypeDefinitions)
        .map((key) => `${key}: ${contentTypeDefinitions[key]}`)
        .join('\n')}
  }
`;
  createTypes(finalDefinitions);
};

[LekoArts]

This feels like a bit of a duplicate of your other issue so I'm just referring to my other answer: #24427 (comment)