Elasticsearch `function_score` with `score_mode` confusion when used with nested objects - elasticsearch

Question

Background:
I have the following mapping for curriculum_posts documents. Notice the nested skills property.
{
"curriculum_posts" : {
"mappings" : {
"dynamic" : "false",
"properties" : {
"title" : {
"type" : "text",
"analyzer" : "english"
},
"skills" : {
"type" : "nested",
"properties" : {
"slug" : {
"type" : "text",
"fields" : {
"raw" : {
"type" : "keyword"
},
"text" : {
"type" : "text"
}
}
},
"start_skill_level" : {
"type" : "keyword"
},
"start_skill_level_value" : {
"type" : "integer"
}
}
}
}
}
}
}
A sample record looks like this:
{
"_source" : {
"skills" : [
{
"start_skill_level_value" : 1,
"slug" : "infrastructure-as-code-iac"
},
{
"start_skill_level_value" : 1,
"slug" : "devops"
}
],
"title" : "Terraform: Infrastructure as code"
}
}
I wanted to run a query that return all documents but with scores matching the number of skills.slug values that matched. My query looks like this:
{
"query": {
"nested": {
"path": "skills",
"query": {
"function_score": {
"query": { "match_all": {} },
"functions": [
{ "script_score": { "script": "0" } },
{
"filter": {
"term": { "skills.slug.raw": { "value": "devops" } }
},
"weight": 2
},
{
"filter": {
"term": { "skills.slug.raw": { "value": "infrastructure-as-code-iac" } }
},
"weight": 2
}
],
"score_mode": "sum",
"boost_mode": "replace"
}
}
}
}
}
I decided to use function_score with boost_mode: replace so that the scores from documents are ignored and only the function scores are taken. The score_mode: sum to ensure that the scores from the function matches are summed up.
The problem
So, for the above query, on the example document above, I was expecting the score to be 4.0 because it matches the skills.slug for both infrastructure-as-code-iac and devops. However, I the score in the result is only 2.0 for the document.
Question
I suppose I'm not understanding how function_score takes the scores from the functions or how my functions are effecting the score. Could someone help me understand the scoring here?
Some debugging
I looked at the explanation but I'm unable to decode much information from it. Nevertheless, here is the explanation:
{
"_index" : "curriculum_posts",
"_type" : "_doc",
"_id" : "18",
"matched" : true,
"explanation" : {
"value" : 2.0,
"description" : "Score based on 2 child docs in range from 83 to 93, best match:",
"details" : [
{
"value" : 2.0,
"description" : "sum of:",
"details" : [
{
"value" : 2.0,
"description" : "min of:",
"details" : [
{
"value" : 2.0,
"description" : "function score, score mode [sum]",
"details" : [
{
"value" : 0.0,
"description" : "script score function, computed with script:\"Script{type=inline, lang='painless', idOrCode='0', options={}, params={}}\"",
"details" : [
{
"value" : 1.0,
"description" : "_score: ",
"details" : [
{
"value" : 1.0,
"description" : "*:*",
"details" : [ ]
}
]
}
]
},
{
"value" : 2.0,
"description" : "function score, product of:",
"details" : [
{
"value" : 1.0,
"description" : "match filter: skills.slug.raw:infrastructure-as-code-iac",
"details" : [ ]
},
{
"value" : 2.0,
"description" : "product of:",
"details" : [
{
"value" : 1.0,
"description" : "constant score 1.0 - no function provided",
"details" : [ ]
},
{
"value" : 2.0,
"description" : "weight",
"details" : [ ]
}
]
}
]
}
]
},
{
"value" : 3.4028235E38,
"description" : "maxBoost",
"details" : [ ]
}
]
},
{
"value" : 0.0,
"description" : "match on required clause, product of:",
"details" : [
{
"value" : 0.0,
"description" : "# clause",
"details" : [ ]
},
{
"value" : 1.0,
"description" : "_type:__skills",
"details" : [ ]
}
]
}
]
}
]
}
}