How to return 0 for requested data in aggregation if no documents matched - elasticsearch

Question

I have an index of users with structure:
User
book_ids:[] //array of book ids
books : [{
book_id:
name:
}] //array of books
I want to create a query that returns a map of Book Id and number of users that read it.
The result of the query should include books that are not used by any user.
I have a very simplified version of the query:
{
"query":{
"bool":{
"must":[
{
"nested":{
"path":"books",
"query": {
"bool": {
"must": {
"terms": {
"books.book_id": [100,200] //book ids that provided as a parameter
}
}
}
}
}
}
]
}
},
"aggs":{
"books":{
"terms":{
"field":"book_ids",
"include":[100,200] //book ids that provided as a parameter
}
}
},
"size":0
}
The result of the query will be
buckets: [
{key: 100, doc_count: 53}
]
So there are 53 users who read the book with id 100, but there is no user who reads book with id 200(as we don't have it in response).
The question here is how can I change the query to get a following result:
buckets: [
{key: 100, doc_count: 53},
{key: 200, doc_count: 0}
]

Answer 1

Terms aggregations doesn't add the bucket in the result if a given term does not exist in the index.
You can use filters aggregation for this purpose:
{
"query": {
...
},
"aggs": {
"books": {
"filters": {
"filters": {
"100": { "match": { "book_ids": 100 } },
"200": { "match": { "book_ids": 200 } }
}
}
}
},
"size": 0
}
To reproduce
# post some books ids, with the 5 missing
POST /_bulk
{ "index" : { "_index" : "72201832" } }
{ "book_ids": [1, 2, 3] }
{ "index" : { "_index" : "72201832" } }
{ "book_ids": [4, 2, 3] }
{ "index" : { "_index" : "72201832" } }
{ "book_ids": [6, 2, 3] }
{ "index" : { "_index" : "72201832" } }
{ "book_ids": [7, 2, 3] }
GET /72201832/_search
{
"size": 0,
"aggs": {
"books": {
"filters": {
"filters": {
"1": { "term": {"book_ids": "1"} },
"2": { "term": {"book_ids": "2"} },
"3": { "term": {"book_ids": "3"} },
"4": { "term": {"book_ids": "4"} },
"5": { "term": {"book_ids": "5"} },
"6": { "term": {"book_ids": "6"} },
"7": { "term": {"book_ids": "7"} }
}
}
}
}
}