Elasticsearch: Aggregate distinct values in array - elasticsearch

Question

I am using Elasticsearch to store click traffic and each row includes topics of the page which has been visited. A typical row looks like:
{
"date": "2017-09-10T12:26:53.998Z",
"pageid": "10263779",
"loc_ll": [
-73.6487,
45.4671
],
"ua_type": "Computer",
"topics": [
"Trains",
"Planes",
"Electric Cars"
]
}
I want each topics to be a keyword so if I search for cars nothing will be returned. Only Electric Cars would return a result.
I also want to run a distinct query on all topics in all rows so I have a list of all topics used.
Doing this on a pageid would look like like the following, but I am unsure how to approach this for the topics array.
{
"aggs": {
"ids": {
"terms": {
"field": pageid,
"size": 10
}
}
}
}

Answer 1

Your approach to querying and getting the available terms looks fine. Probably you should check your mapping. If you get results for cars this looks as your mapping for topics is an analyzed string (e.g. type text instead of keyword). So please check your mapping for this field.
PUT keywordarray
{
"mappings": {
"item": {
"properties": {
"id": {
"type": "integer"
},
"topics": {
"type": "keyword"
}
}
}
}
}
With this sample data
POST keywordarray/item
{
"id": 123,
"topics": [
"first topic", "second topic", "another"
]
}
and this aggregation:
GET keywordarray/item/_search
{
"size": 0,
"aggs": {
"topics": {
"terms": {
"field": "topics"
}
}
}
}
will result in this:
"aggregations": {
"topics": {
"doc_count_error_upper_bound": 0,
"sum_other_doc_count": 0,
"buckets": [
{
"key": "another",
"doc_count": 1
},
{
"key": "first topic",
"doc_count": 1
},
{
"key": "second topic",
"doc_count": 1
}
]
}
}

Answer 2

It is very therapeutic asking on SO. Simply changing the mapping type to keyword allowed me to achieve what I needed.
A part of me thought that it would concatenate the array into a string. But it doesn't
{
"mappings": {
"view": {
"properties": {
"topics": {
"type": "keyword"
},...
}
}
}
}
and a search query like
{
"aggs": {
"ids": {
"terms": {
"field": pageid,
"size": 10
}
}
}
}
Will return a distinct list of all elements in a fields array.