Unique slugs for nested projects within clients in Sanity schema

// schemas/client.js
export default {
  name: 'client',
  type: 'document',
  fields: [
    {
      name: 'name',
      type: 'string',
      validation: Rule => Rule.required()
    },
    {
      name: 'slug',
      type: 'slug',
      options: {
        source: 'name'
      },
      validation: Rule => Rule.required()
    }
  ]
}

// schemas/project.js
export default {
  name: 'project',
  type: 'document',
  fields: [
    {
      name: 'title',
      type: 'string',
      validation: Rule => Rule.required()
    },
    {
      name: 'client',
      type: 'reference',
      to: [{type: 'client'}],
      validation: Rule => Rule.required()
    },
    {
      name: 'slug',
      type: 'slug',
      options: {
        source: 'title'
      },
      validation: Rule => Rule.required().custom(async (slug, context) => {
        const {document, getClient} = context
        const client = getClient({apiVersion: '2023-01-01'})
        
        if (!slug?.current || !document?.client?._ref) {
          return true // Let required validation handle empty values
        }
        
        const query = `count(*[
          _type == "project" && 
          slug.current == $slug && 
          client._ref == $clientRef &&
          _id != $id
        ])`
        
        const params = {
          slug: slug.current,
          clientRef: document.client._ref,
          id: document._id
        }
        
        const count = await client.fetch(query, params)
        
        return count === 0 ? true : 'This slug is already used by another project for this client'
      })
    }
  ]
}

// structure/index.js
export const structure = (S) =>
  S.list()
    .title('Content')
    .items([
      S.listItem()
        .title('Clients')
        .child(
          S.documentTypeList('client')
            .title('Clients')
            .child((clientId) =>
              S.documentList()
                .title('Projects')
                .filter('_type == "project" && client._ref == $clientId')
                .params({clientId})
            )
        ),
      S.divider(),
      ...S.documentTypeListItems()
    ])

*[_type == "project"] {
  title,
  "slug": slug.current,
  "clientSlug": client->slug.current,
  "fullPath": client->slug.current + "/" + slug.current
}