Query Sum of Nested Array-Length COSMOS DB - azure-cosmosdb

Question

Assume this model:
Having a collection of Writers documents, each Writer has some Posts. and each Post contains an array of Comments.
JSON:
{
"id": "1",
"partitionKey": "somePK",
"name": "John",
"posts": [
{
"id": "20",
"title": "post1",
"comments": [
{
"body": "some body"
}
]
},
{
"id": "21",
"title": "post2",
"comments": [
{
"body": "some new body"
}
]
}
]
}
I need a query that returns the following output:
[
{
"WriterName": "John",
"WriterCommentsCount": 2
}
]
I managed to get the writer's comment but I have a problem getting the name(or other properties) beside the WriterCommentsCount. Any idea of how to get the writer's name?
this is what I've tried so far (only the writer's comments count)

Answer 1

Along your thought, if we execute sql
SELECT c.name,array_length(post.comments) as countC FROM c join post in c.posts where c.id = '1'
we'll get response like below:
[
{
"name": "John",
"countC": 1
},
{
"name": "John",
"countC": 3
},
{
"name": "John",
"countC": 0
}
]
So using this SELECT c.name,sum(array_length(post.comments)) as countC FROM c join post in c.posts where c.id = '1' will return error 'Property reference 'c.name' is invalid'. The solution is using 'group by c.name'.
My answer in the comment and the one from #404 are all the same in this point, the difference is that I followed your thought using sum function and he uses count(1) after expand all the child item. My sql costs '3.09 RUs' and his costs the same on my test data.
Here's the situation screenshot.

Answer 2

The easiest is making a flat structure where you have an entry for each comment. After that you can use GROUP BY to group all entries for the same name. Try the following:
SELECT
c.name AS WriterName,
COUNT(1) AS WriterCommentsCount
FROM c
JOIN s IN c.posts
JOIN t IN s.comments
WHERE c.id = '1'
GROUP BY c.name