Elasticsearch indexing homogenous objects under dynamic keys - elasticsearch

Question

The kind of document we want to index and query contains variable keys but are grouped into a common root key as follows:
{
"articles": {
"0000000000000000000000000000000000000001": {
"crawled_at": "2016-05-18T19:26:47Z",
"language": "en",
"tags": [
"a",
"b",
"d"
]
},
"0000000000000000000000000000000000000002": {
"crawled_at": "2016-05-18T19:26:47Z",
"language": "en",
"tags": [
"b",
"c",
"d"
]
}
},
"articles_count": 2
}
We want to able to ask: what documents contains articles with tags "b" and "d", with language "en".
The reason why we don't use list for articles, is that elasticsearch can efficiently and automatically merge documents with partial updates. The challenge however is to index the objects inside under the variable keys. One possible way we tried is to use dynamic_templates as follows:
{
"sources": {
"dynamic": "strict",
"dynamic_templates": [
{
"article_template": {
"mapping": {
"fields": {
"crawled_at": {
"format": "dateOptionalTime",
"type": "date"
},
"language": {
"index": "not_analyzed",
"type": "string"
},
"tags": {
"index": "not_analyzed",
"type": "string"
}
}
},
"path_match": "articles.*"
}
}
],
"properties": {
"articles": {
"dynamic": false,
"type": "object"
},
"articles_count": {
"type": "integer"
}
}
}
}
However this dynamic template fails because when documents are inserted, the following can be found in the logs:
[2016-05-30 17:44:45,424][WARN ][index.codec] [node]
[main] no index mapper found for field:
[articles.0000000000000000000000000000000000000001.language] returning
default postings format
Same for the two other fields as well. When I try to query for the existence of a certain article, or even articles it doesn't return any document (no error but empty hits):
curl -LsS -XGET 'localhost:9200/main/sources/_search' -d '{"query":{"exists":{"field":"articles"}}}'
When I query for the existence of articles_count, it returns everything. Is there a minor error in what we are trying to achieve, for example in the schema: the definition of articles as a property and in the dynamic template? What about the types and dynamic false? The path seems correct. Maybe this is not possible to define templates for objects in variable-keys, but it should be according to the documentation.
Otherwise, what alternatives are possible without changing the document if possible?
Notes: we have other types in the same index main that also have these fields like language, I ignore if it could influence. The version of ES we are using is 1.7.5 (we cannot upgrade to 2.X for now).