Aggregate by property on parent document with Elasticsearch join field - elasticsearch

Question

I have an Elasticsearch index that uses a join type field to relate two types of indexed documents to each other via a parent-child relation: posts which are parents of comments.
posts have a category keyword field, and comments belong to posts. I would like to find the number of comments in each post category, like so:
// what query do I need to get this result?
{
"aggregations" : {
"comment-counts-by-post-category" : {
"buckets" : [
{
"key" : "Dogs",
"doc_count" : 2,
},
{
"key" : "Cats",
"doc_count" : 1,
}
]
}
}
}
Here is a complete example:
I have an index with the following mapping:
PUT posts-index/
{
"mappings": {
"properties": {
"post": {
"type": "object",
"properties": {
"category": {
"type": "keyword"
}
}
},
"text": {
"type": "keyword"
},
"post_comment_join": {
"type": "join",
"relations": {
"post": "comment"
}
}
}
}
}
I create two posts, one in the Dogs category, and one in the Cats category:
PUT posts-index/_doc/post-1
{
"text": "this is a dog post",
"post": {
"category": "Dogs"
},
"post_comment_join": {
"name": "post"
}
}
PUT posts-index/_doc/post-2
{
"text": "this is a cat post",
"post": {
"category": "Cats"
},
"post_comment_join": {
"name": "post"
}
}
Then, I create a few comments (in this case, 2 on the dog post and 1 on the cat post)
PUT posts-index/_doc/comment-1&routing=1&refresh
{
"text": "this is comment 1 for post 1",
"post_comment_join": {
"name": "comment",
"parent": "post-1"
}
}
PUT posts-index/_doc/comment-2&routing=1&refresh
{
"text": "this is comment 2 for post 1",
"post_comment_join": {
"name": "comment",
"parent": "post-1"
}
}
PUT posts-index/_doc/comment-3&routing=1&refresh
{
"text": "this is a comment 1 for post 2",
"post_comment_join": {
"name": "comment",
"parent": "post-2"
}
}
I can search for all comment documents using a has_parent query:
POST post-index/_search
{
"query": {
"has_parent": {
"parent_type": "post",
"query": {
"match_all": {}
}
}
}
}
{
"hits": {
"total": {
"value": 3,
"relation": "eq"
},
"max_score": 1.0,
"hits": [ /* returns the 3 comments */ ]
}
}
What I can't figure out how to do is find the number of comments in each category
I've looked into Parent Aggregations, but they seem to only allow you aggregate based on the type of the parent. In this case, all parents are of type post, so that doesn't help.
I've also tried using a basic terms aggregation using the join_field#parent_field syntax:
POST post-index/_search
{
"query": {
"has_parent": {
"parent_type": "post",
"query": {
"match_all": {}
}
}
},
"aggs": {
"agg-by-post-category": {
"terms": {
"field": "post_comment_join#post.category"
}
}
}
}
// returns { "buckets": [] } in the aggs
Unfortunately, this returns no results. It seems as though the post_comment_join#post syntax can be used to aggregate by parent doc, but not by an attribute on the parent doc. (i.e., by the _id field of a post, but not by post.category)
Can anyone help me figure out the right aggs syntax to return all comments grouped by their parent post's category?
Again, here is the result I'm looking for:
{
"aggregations" : {
"comment-counts-by-post-category" : {
"buckets" : [
{
"key" : "Dogs",
"doc_count" : 2,
},
{
"key" : "Cats",
"doc_count" : 1,
}
]
}
}
}
Platform details
Amazon Opensearch service version 7.9

Answer 1

You can use any of below two to find count of comments by category.
GET posts-index/_search
{
"query": {
"has_child": {
"type": "comment",
"inner_hits": {
"_source": false,
"size": 0
},
"query": {
"match_all": {}
}
}
}
}
GET posts-index/_search
{
"aggs": {
"top-tags": {
"terms": {
"field": "post.category",
"size": 10
},
"aggs": {
"to-answers": {
"children": {
"type": "comment"
},
"aggs": {
"comments-count": {
"value_count": {
"field": "text"
}
}
}
}
}
}
}
}