Sorting a terms aggregation by sub-terms-aggregation cardinality aggregation

[liamwhite]

I'd like to be able to order a terms aggregation by the value of a child terms aggregation. Can this be done?

Here's what I have so far:

:vote_stats:
  :size: 0
  :aggs:
    :ip_votes:
      :terms: { field: 'ip', order: {'images.users.value': 'desc'} } # <- can't do this
      :aggs:
        :images:
          :terms: { field: 'image_id', order: {'users.value': 'desc'} }
          :aggs:
            :users:
              :cardinality: { field: 'user_id' }
            :selector:
              :bucket_selector: { buckets_path: { value: 'users.value' }, script: 'params.value > 1' }

If I can't do that, can I at least filter the ip_votes aggregation to buckets where the images sub-aggregation isn't empty, as occurs if the order statement was left blank?

Snipped example of current output (the images buckets are empty, I want to find the images buckets that aren't empty):

{
  "doc_count_error_upper_bound": 228,
  "sum_other_doc_count": 58319,
  "buckets": [
    {
      "key": "xx.xx.94.189",
      "doc_count": 3005,
      "images": {
        "doc_count_error_upper_bound": -1,
        "sum_other_doc_count": 2995,
        "buckets": [

        ]
      }
    },
    {
      "key": "xx.xx.116.134",
      "doc_count": 1895,
      "images": {
        "doc_count_error_upper_bound": -1,
        "sum_other_doc_count": 1885,
        "buckets": [

        ]
      }
    },
    {
      "key": "xxxx:xxxx:xxxx:xxxx:c169:fa2b:1eed:6153",
      "doc_count": 1536,
      "images": {
        "doc_count_error_upper_bound": -1,
        "sum_other_doc_count": 1526,
        "buckets": [

        ]
      }
    }
}