Example of function_score using the script_score function. This achieves a result similar to creating a script field and then sorting descending by that dynamic value.

script-score.sh

#!/bin/sh

curl -XDELETE localhost:9200/foo >/dev/null

curl -XPOST localhost:9200/foo/t -d'
{
    "clicks": 10, 
    "impressions": 1000, 
    "when": "2013-10-01"
}
'; echo

curl -XPOST localhost:9200/foo/t -d'
{
    "clicks": 27, 
    "impressions": 1488, 
    "when": "2013-10-22"
}
'; echo

curl -XPOST localhost:9200/foo/t -d'
{
    "clicks": 4, 
    "impressions": 500, 
    "when": "2013-10-23"
}
'; echo

curl -XPOST localhost:9200/foo/t -d'
{
    "clicks": 1282, 
    "impressions": 100000, 
    "when": "2013-11-01"
}
'; echo

curl -XPOST localhost:9200/foo/_refresh; echo

curl localhost:9200/foo/_search -d@- <<EOF
{
    "query": {
        "filtered": {
            "filter": {
                "range": {
                    "when": {
                        "gte": "2013-10-20", 
                        "lte": "2013-10-25"
                    }
                }
            }, 
            "query": {
                "function_score": {
                    "boost_mode": "replace", 
                    "functions": [
                        {
                            "script_score": {
                                "script": "doc['clicks'].value / doc['impressions'].value"
                            }
                        }
                    ], 
                    "query": {
                        "match_all": {}
                    }
                }
            }
        }
    }
}
EOF
echo

## Note that _score is the result of the script (automatically sorted
## greatest->least)
##
## {
##     "_shards": {
##         "failed": 0, 
##         "successful": 5, 
##         "total": 5
##     }, 
##     "hits": {
##         "hits": [
##             {
##                 "_id": "AU7EdQF0RfqGh1ceKRfCnA", 
##                 "_index": "foo", 
##                 "_score": 0.01814516, 
##                 "_source": {
##                     "clicks": 27, 
##                     "impressions": 1488, 
##                     "when": "2013-10-22"
##                 }, 
##                 "_type": "t"
##             }, 
##             {
##                 "_id": "eN66w46CSfeYLf_mZ8le0Q", 
##                 "_index": "foo", 
##                 "_score": 0.008, 
##                 "_source": {
##                     "clicks": 4, 
##                     "impressions": 500, 
##                     "when": "2013-10-23"
##                 }, 
##                 "_type": "t"
##             }
##         ], 
##         "max_score": 0.01814516, 
##         "total": 2
##     }, 
##     "timed_out": false, 
##     "took": 3
## }