Why script in custom_filters_score behaves as boost? - elasticsearch

Question

{
"query": {
"custom_filters_score": {
"query": {
"term": {
"name": "user1234"
}
},
"filters": [
{
"filter": {
"term": {
"subject": "math"
}
},
"script": "_score + doc['subject_score'].value"
}
]
}
}
}
If script is having like above it gives Error: unresolvable property or identifier: _score
If script is like "script": "doc['subject_score'].value" It multiplies the _score in similar way boost does. I want to replace the elasticsearch _score with custom score.

Answer 1

If I understood you correctly you would like to use elasticsearch scoring if subject is not math and you would like to use custom scoring with subject is math. If you are using Elasticsearch v0.90.4 or higher, it can be achieved using new function_score query:
{
"query": {
"function_score": {
"query": {
"term": {
"name": "user1234"
}
},
"functions": [{
"filter": {
"term": {
"subject": "math"
}
},
"script_score": {
"script": "doc[\"subject_score\"].value"
}
}, {
"boost_factor": 0
}],
"score_mode": "first",
"boost_mode": "sum"
}
}
}
Prior to v0.90.4 you would have to resort to using combination of custom_score and custom_filters_score:
{
"query": {
"custom_score": {
"query": {
"custom_filters_score": {
"query": {
"term": {
"name": "user1234"
}
},
"filters": [{
"filter": {
"term": {
"subject": "math"
}
},
"script": "-1.0"
}]
}
},
"script": "_score < 0.0 ? _score * -1.0 + doc[\"subject_score\"].value : _score"
}
}
}
or as #javanna suggested, use multiple custom_score queries combined together by bool query:
{
"query": {
"bool": {
"disable_coord": true,
"should": [{
"filtered": {
"query": {
"term": {
"name": "user1234"
}
},
"filter": {
"bool": {
"must_not": [{
"term": {
"subject": "math"
}
}]
}
}
}
}, {
"filtered": {
"query": {
"custom_score": {
"query": {
"term": {
"name": "user1234"
}
},
"script": "doc['subject_score'].value"
}
},
"filter": {
"term": {
"subject": "math"
}
}
}
}]
}
}
}

Answer 2

Firstly I'd like to say that there are many ways of customising the scoring in elasticsearch and it seems like you may have accidentally picked the wrong one. I will just summarize two and you will see what the problem is:
Custom Filters Score
If you read the docs (carefully) on custom_filters_score then you will see that it there for performance reasons, to be able to use for scoring the the faster filter machinery of elasticsearch. (Filters are faster as scoring is not calculated when computing the hit set, and they are cached between requests.)
At the end of the docs; it mentions custom_filters_score can take a "script" parameter to use instead of a "boost" parameter per filter. Best way to think of this is to calculate a number, which will be passed up to the parent query to be combined with the other sibling queries to calculate the total score for the document.
Custom Score Query
Reading the docs this is used when you want to customise the score from the query and change it how you wish. There is a _score variable available to you to use in your "script" which is the score of the query inside the custom_score query.
Try this:
"query": {
"filtered": {
"query": {
"custom_score": {
"query": {
"match_all": {}
},
"script": "doc['subject_score'].value" //*see note below
}
},
"filter": {
"and": [
{
"term": {
"subject": "math"
}
},
{
"term": {
"name": "user1234"
}
}
]
}
}
}
*NOTE: If you wanted to you could use _score here. Also, I moved both your "term" parts to filters as any match of a term would get the same score and filters are faster.
Good luck!