min_score not working in function_score query

[clintongormley]

The new min_score functionality in the function_score query doesn't seem to to work as advertised. For instance:

DELETE t

PUT t
{
  "mappings": {
    "t": {
      "properties": {
        "name": {
          "type": "string"
        },
        "foo": {
          "type": "nested",
          "properties": {
            "key": {
              "type": "string",
              "index": "not_analyzed"
            },
            "value": {
              "type": "long"
            }
          }
        }
      }
    }
  }
}

PUT /t/t/1
{
  "name": "test",
  "foo": [
    {
      "key": "bar",
      "value": 10
    },
    {
      "key": "bar",
      "value": 20
    },
    {
      "key": "bar",
      "value": 30
    },
    {
      "key": "bar",
      "value": 40
    }
  ]
}

This query returns a score of 4 for the above document, which means that it should be filtered out by the min_score of 10:

GET /_search
{
  "query": {
    "function_score": {
      "boost_mode": "replace",
      "min_score": 10,
      "query": {
        "nested": {
          "path": "foo",
          "score_mode": "sum",
          "query": {
            "constant_score": {
              "filter": {
                "match_all": {}
              }
            }
          }
        }
      }
    }
  }
}

[brwe]

This is a bug indeed and it seems to me for two reasons:

1. when the score is 4 the document should not be returned
2. the score should not be 4 in the first place. "boost_mode": "replace" should cause the score to be 1 (default if no functions and no query given).
   I'll work on a fix. For now the workaround would be to define a

"weight": 1,

if you really want to score all matches with 1 (which does not make much sense imo but should still work) or

 "script_score": {
        "script": "_score"
}, 

in case one actually wants the score of the query to be used.

[pickypg]

To be clear, min_score as a child of function_score is new in 1.5.0.

min_score at the root document level works locally running 1.4.4.

[brwe]

Currently the way it works is this: If function_score encounters a query without a function, then the query is just executed as is without wrapping in a function score query (https://github.com/elastic/elasticsearch/blob/master/src/main/java/org/elasticsearch/index/query/functionscore/FunctionScoreQueryParser.java#L161) so min_score has no effect in this case. This was implemented so on purpose but it seems to me that now with the min_score there is a usecase where you would want to use function_score without a function. I'll change that.