Example of listing documents in Sanity Studio‘s desk structure with real-time listener

department.js

export default {
  name: 'department',
  type: 'document',
  title: 'Department',
  fields: [
    {
      name: 'title',
      type: 'string',
      title: 'Title',
    },
    {
      name: 'sites',
      type: 'array',
      title: 'Sites',
      of: [
        {
          type: 'reference',
          to: [{type: 'site'}]
        }
      ]
    }
  ]
}

deskStructure.js

import S from '@sanity/desk-tool/structure-builder'
import documentStore from 'part:@sanity/base/datastore/document'
// remember to add rxjs/operators to your studio dependencies
import { map } from 'rxjs/operators'

// filter down documents of the type “site” that has its _id in the selected department
// parent document array of sites
const query = `*[_type == $type && _id in *[_id == $parentId][0].sites[]._ref]`

export default () =>
  S.list()
    .title('Content')
    .items([
      S.listItem()
        .title('Sites by department')
        .child(
          S.documentList()
            .title('Department')
            .filter('_type == $type')
            .params({ type: 'department' })
            .child(parentId =>
              // set up a listener to make list real-time
              documentStore
                .listenQuery(
                  query, {type: 'site', parentId}
                )
                .pipe(
                  map(sites =>
                    S.documentList()
                      .title('Sites')
                      .filter('_id in $ids')
                      .params({ids: sites.map(({_id}) => _id)})
                  )
                )
            )
        ),

      ...S.documentTypeListItems()
    ])

site.js

export default {
  name: 'site',
  type: 'document',
  title: 'Site',
  fields: [
    {
      name: 'title',
      type: 'string',
      title: 'Title'
    }
  ]
}