ElasticSearch Script Score Using Field Value - elasticsearch

Question

ElasticSearch 1.2.1
I am trying to query documents using weighted tags.
curl -X PUT 'http://localhost:9200/test'
curl -X PUT 'http://localhost:9200/test/thing/_mapping' - '{
"thing": {
"properties": {
"tags": {
"type": "nested",
"properties": {
"name": { "type": "string" },
"weight": { "type": "integer" }
}
}
}
}}'
Adding a document:
curl -X PUT 'http://localhost:9200/test/thing/1', -d '{
"tags": [
{ "name": "a", "weight": 2 }
]
}'
Now I am searching for documents having a tag a and boost the score based on the weight.
Note: to run these examples you have to enable dynamic scripting in ElasticSearch: add script.disable_dynamic: false to elasticsearch.yml
curl -X GET 'http://localhost:9200/test/thing/_search?pretty' -d '{
"query": {
"function_score": {
"boost_mode": "replace",
"query": {
"match_all": {}
},
"functions": [
{
"filter": {
"nested": {
"path": "tags",
"filter": {
"term": {
"tags.name": "a"
}
}
}
},
"script_score": {
"script": "doc.weight.value"
}
}
]
}
}
}'
The document is found, as expected, however the score is 0. It seems as if the property doc.weight was empty.
Let's test this by replacing the script with doc.weight.empty ? 50 : 100. The hit now has a score of 50 indicating that the field doc.weight is empty. It is found though, because using a non-existant field name (e.g. doc.foobar) gives an error.
Background: The match_all part would be replaced by a real query. I want to use the tags to boost results matching the tags before the ones not matching the tags.
What am I missing?