Can't Highlight Dynamic Template Field Value in Elasticsearch - elasticsearch

Question

Follow up to this question.
I have a dynamic template which copies the text of a JSON blob to a single text field, and I'd like to search on that field and highlight matches. Here is my full code for ES 6.5
DELETE /test
PUT /test?include_type_name=true
{
"settings": {"number_of_shards": 1,"number_of_replicas": 1},
"mappings": {
"_doc": {
"dynamic_templates": [
{
"full_name": {
"match_mapping_type": "string",
"path_match": "content.*",
"mapping": {
"type": "text",
"copy_to": "content_text"
}
}
}
],
"properties": {
"content_text": {
"type": "text"
},
"content": {
"type": "object",
"enabled": "true"
}
}
}
}
}
PUT /test/_doc/1?refresh=true
{
"content": {
"a": {
"b": {
"text": "42"
}
}
}
}
GET /test/_search
{
"query": {
"match": {
"content_text": "42"
}
},
"highlight": {
"fields": {
"content_text": {}
}
}
}
The response does not show the highlighted content_text
{
"took" : 0,
"timed_out" : false,
"_shards" : {
"total" : 1,
"successful" : 1,
"skipped" : 0,
"failed" : 0
},
"hits" : {
"total" : 1,
"max_score" : 0.2876821,
"hits" : [
{
"_index" : "test",
"_type" : "_doc",
"_id" : "1",
"_score" : 0.2876821,
"_source" : {
"content" : {
"a" : {
"b" : {
"text" : "42"
}
}
}
}
}
]
}
}
As you can see, the content_text field is not highlight. It's also not in the response at all. How do I get highlights for this field to show up?

Answer 1

This is a tricky one, but will make sense once you read what follows.
As per the official documentation on highlighting, the actual content of a field is required to exist somewhere. So if the field is not stored (i.e. the mapping does not set store to true), the actual _source is loaded and the relevant field is extracted from _source.
In your case, the content_text field doesn't exist in the _source document (i.e. it is just indexed from other text fields present in content.*) and in the mapping, the store parameter is not set to true (it is false by default).
So you simply need to change your mapping to this:
"content_text": {
"store": true,
"type": "text"
},
And then your query will yield this:
"highlight" : {
"content_text" : [
"<em>42</em>"
]
}