Add support of field boost in prefix query

[spigene]

ES ignores field level boost factors specified in mapping when computing documents' score for query string rewritten to prefix query.

Sample setup:

1. Create index and mapping.

curl -XPUT 'http://localhost:9200/messages/'

curl -XPUT 'http://localhost:9200/messages/message/_mapping' -d '
{
    "message" : {
        "properties" : {
            "message" : {"type" : "string", "store" : true },
            "comment" : {"type" : "string", "store" : true , "boost" : 5.0 }
        }
    }
}'

2. Create sample docs

curl -XPUT 'http://localhost:9200/messages/message/1' -d '{
    "user" : "user1",
    "message" : "test message",
    "comment" : "whatever"
}'

curl -XPUT 'http://localhost:9200/messages/message/2' -d '{
    "user" : "user2",
    "message" : "hello world",
    "comment" : "test comment"
}'

3. Wait for ES to be synced

curl -XPOST 'http://localhost:9200/messages/_refresh' 

4. Search in default catch-all field using query string rewritten to term query

curl -XPOST 'http://localhost:9200/messages/_search' -d '{ "query" : { "query_string" : { "query" : "test" } } , "explain" : true }' | python -mjson.tool

5. Search in default catch-all field using query string rewritten to prefix query

curl -XPOST 'http://localhost:9200/messages/_search' -d '{ "query" : { "query_string" : { "query" : "tes*" , "rewrite" : "top_terms_boost_10" } } , "explain" : true }' | python -mjson.tool

The first query (step 4) works as expected -- document with id equal to 2 has higher score.
The second query (step 5) returns two documents with equal score.

[crowleym]

@s1monw would your fix for #5258 also apply in this case?
Thanks.

[clintongormley]

Hi @ei82

Sorry it has taken a while to get to this issue. You are using the wrong rewrite method - instead of top_terms_boost_10 (which applies a constant boost) you want top_terms_10, which takes the actual term weights into account.

See http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-multi-term-rewrite.html