Filtering sub-documents in Sanity.io using the @ operator

# example: every series has a volume. Some volumes have a download link and some do not
# query every series, but for each series keep only the volumes whose download_link field is not empty/null/undefined

*[
  _type == "series" &&
  // defined(volumes[] -> download_link) // should the filtering be done here? Is this the right approach?
] {
  _id,
  title,
  volumes[] -> {     // or does the filtering have to be done here?
    _id,
    title,
    download_link,
  }
}

*[
  _type == "series" &&
] {
  _id,
  title,
  volumes[defined(@->.download_link)] -> { 
    _id,
    title,
    download_link,
  }
}

# these work on sanity studio, but not in my project even if I copy/paste verbatim
volumes[defined(@->.download_link)] -> { /// rest of the query
volumes[defined(@["download_link"])] -> { /// rest of the query

# tried these as a hail mary solution but none worked.
volumes[defined(@[]->.download_link)] -> { /// rest of the query
volumes[defined(@.download_link)] -> { /// rest of the query

export async function getDownloads(): Promise<Release[]> {
  const data: Release[] = await client.fetch(`
    *[
      _type == "series" && 
      length(volumes[]) > 0
    ] {
      _id, 
      title,
      volumes[] -> {
        _id,
        title,
        download_link,
      }
    }
  `);
  return data;
}