Example of how to query multi level nested documents in Sanity

category.js

import { FcTreeStructure } from 'react-icons/fc';

export default {
  name: 'category',
  title: 'Category',
  type: 'document',
  icon: FcTreeStructure,
  fields: [
    {
      name: 'title',
      title: 'Title',
      type: 'string',
    },
    {
      name: 'description',
      title: 'Description',
      type: 'text',
    },
    {
      name: 'child',
      title: 'Child Category',
      type: 'array',
      of: [{ type: 'reference', to: [{ type: 'category' }] }],
    },
  ],
  preview: {
    select: {
      title: 'title',
    },
  },
};

nested.js

const sanityClient = require('@sanity/client');
const groq = require('groq');
const fs = require('fs');
const prettier = require('prettier');

function getMultiLevelCategoryQuery(index, max) {
  const childQuery = index < max ? getMultiLevelCategoryQuery(index + 1, max) : '...';
  return `
  title,
  "products": *[_type=='product' && references(^._id)]{ title },
  child[]->{
    ${childQuery}
  }
`;}

async function getMultiLevelCategories(client) {
  const query = groq`
    *[_type == "category" && title == "Level 1"] {
      ${getMultiLevelCategoryQuery(0, 10)}
    }
  `;
  const data = await client.fetch(query);
  return data;
}

const options = {
  dataset: '<your-dataset>',
  projectId: '<your-project-id>',
  useCdn: false, // just a demo so no need for cdn obv
};

const client = sanityClient(options);

module.exports = (async () => {
  const [categories, prettierConfig] = await Promise.all([
    getMultiLevelCategories(client),
    prettier.resolveConfig('./.prettierrc.js'), // get your local prettier config to make pretty formatting
  ]);
  const formatted = prettier.format(JSON.stringify(categories), {
    ...prettierConfig,
    parser: 'json',
  });
  fs.writeFileSync('./demo.json', formatted);
})();

product.js

import { FcTreeStructure } from 'react-icons/fc';

export default {
  name: 'product',
  title: 'Product',
  type: 'document',
  icon: FcTreeStructure,
  fields: [
    {
      name: 'title',
      title: 'Title',
      type: 'string',
    },
    {
      name: 'description',
      title: 'Description',
      type: 'text',
    },
    {
      name: 'category',
      title: 'Category',
      type: 'array',
      of: [{ type: 'reference', to: [{ type: 'category' }] }],
    },
  ],
  preview: {
    select: {
      title: 'title',
    },
  },
};