Elasticsearch equivalent to SelectMany - elasticsearch

Question

I have a nested type in the Elasticsearch index:
class Category
{
public string Name;
public Category Child;
public Guid[] ProductIds;
}
For example:
Category1
|_Category2
|_Category3
|_Category4
Category3 and Category4 have 2 products each.
What is the right NEST query to get all 4 product ids by searching "Category3"?
I imaging something like this, but can't actually get it work:
var response = await elasticClient.SearchAsync<Category>(s => s
.StoredFields(sf => sf
.Fields(f => f.ProductIds)
)
.Query(q => q
.Nested(n => n
.Path(p => p.Child)
.Query(mu => mu
.Bool(b => b
.Must(m => m
.Match(m => m
.Query("Category Name")
)
)
)
)
)
)
)

Answer 1

One of the options could be to create a sort of breadcrumb hierarchy column and use anlyzer & facet to search over it.
So index can have similar settings for column & analyzer
"btb_breadcrumb": {
"type": "text",
"fields": {
"facet": {
"type": "text",
"analyzer": "custom_path_tree",
"fielddata": "true"
}
}
},
"settings" : {
"analysis": {
"analyzer": {
"english_exact": {
"tokenizer": "standard",
"filter": [
"lowercase"
]
},
"custom_path_tree": {
"tokenizer": "custom_hierarchy"
},
"custom_path_tree_reversed": {
"tokenizer": "custom_hierarchy_reversed"
}
},
"tokenizer": {
"custom_hierarchy": {
"type": "path_hierarchy",
"delimiter": ">"
},
"custom_hierarchy_reversed": {
"type": "path_hierarchy",
"delimiter": ">",
"reverse": "true"
}
}
}
}
and then you can search on this column like this
{
"bool" :
{
"must" :
[
{"match" : {"btb_breadcrumb.facet" : "catid1>catid2>"}}
]
}
}
now this will give you all items with catid2 and all its child categories. Similarly if you want to get all items that fall under any children of catid1, then you can use following query.
{
"bool" :
{
"must" :
[
{"match" : {"btb_breadcrumb.facet" : "catid1>"}}
]
}
}