Elasticsearch - Sort results of Terms aggregation by key string length - sorting

Question

I am querying ES with a Terms aggregation to find the first N unique values of a string field foo where the field contains a substring bar, and the document matches some other constraints.
Currently I am able to sort the results by the key string alphabetically:
{
"query": {other constraints},
"aggs": {
"my_values": {
"terms": {
"field": "foo.raw",
"include": ".*bar.*",
"order": {"_key": "asc"},
"size": N
}
}
}
}
This gives results like
{
...
"aggregations": {
"my_values": {
"doc_count_error_upper_bound": 0,
"sum_other_doc_count": 145,
"buckets": [
{
"key": "aa_bar_aa",
"doc_count": 1
},
{
"key": "iii_bar_iii",
"doc_count": 1
},
{
"key": "z_bar_z",
"doc_count": 1
}
]
}
}
}
How can I change the order option so that the buckets are sorted by the length of the strings in the foo key field, so that the results are like
{
...
"aggregations": {
"my_values": {
"doc_count_error_upper_bound": 0,
"sum_other_doc_count": 145,
"buckets": [
{
"key": "z_bar_z",
"doc_count": 1
},
{
"key": "aa_bar_aa",
"doc_count": 1
},
{
"key": "iii_bar_iii",
"doc_count": 1
}
]
}
}
}
This is desired because a shorter string is closer to the search substring so is considered a 'better' match so should appear earlier in the results than a longer string.
Any alternative way to sort the buckets by how similar they are to the original substring would also be helpful.
I need the sorting to occur in ES so that I only have to load the top N results from ES.

Answer 1

I worked out a way to do this.
I used a sub-aggregation per dynamic bucket to calculate the length of the key string as another field.
Then I was able to sort by this new length field first, then by the actual key so keys of the same length are sorted alphabetically.
{
"query": {other constraints},
"aggs": {
"my_values": {
"terms": {
"field": "foo.raw",
"include": ".*bar.*",
"order": [
{"key_length": "asc"},
{"_key": "asc"}
],
"size": N
},
"aggs": {
"key_length": {
"max": {"script": "doc['foo.raw'].value.length()" }
}
}
}
}
}
This gave me results like
{
...
"aggregations": {
"my_values": {
"doc_count_error_upper_bound": 0,
"sum_other_doc_count": 145,
"buckets": [
{
"key": "z_bar_z",
"doc_count": 1
},
{
"key": "aa_bar_aa",
"doc_count": 1
},
{
"key": "dd_bar_dd",
"doc_count": 1
},
{
"key": "bbb_bar_bbb",
"doc_count": 1
}
]
}
}
}
which is what I wanted.