How to sort an object by it's child object fields? - elasticsearch

Question

How could I sort products by GroupModel.PeerOrder but only if GroupModel.ParentGroupId matches some id?
My C# models:
public class ProductModel
{
public int Id { get; set; }
public int Title { get; set; }
public List<GroupModel> Groups { get; set; }
}
public class GroupModel
{
public int Id { get; set; }
public int Title { get; set; }
public int ParentGroupId { get; set; }
public int PeerOrder { get; set; }
}

Answer 1

Use Nested Query to filter matching "GroupModel.ParentGroupId" values and then apply Nested Sort Query to sort results by "GroupModel.PeerOrder".
As per documentation:
Nested Query: Nested query allows to query nested objects / docs (see nested mapping). The query is executed against the nested objects / docs as if they were indexed as separate docs (they are, internally) and resulting in the root parent doc (or parent nested mapping). Here is a sample mapping:
PUT /my_index
{
"mappings": {
"_doc" : {
"properties" : {
"obj1" : {
"type" : "nested"
}
}
}
}
}
GET /_search
{
"query": {
"nested" : {
"path" : "obj1",
"score_mode" : "avg",
"query" : {
"bool" : {
"must" : [
{ "match" : {"obj1.name" : "blue"} },
{ "range" : {"obj1.count" : {"gt" : 5}} }
]
}
}
}
}
}
Nested Sort Query: It is possible to sort by the value of a nested field, even though the value exists in a separate nested document.
GET /_search
{
"query": {
"nested": {
"path": "comments",
"filter": {
"range": {
"comments.date": {
"gte": "2014-10-01",
"lt": "2014-11-01"
}
}
}
}
},
"sort": {
"comments.stars": {
"order": "asc",
"mode": "min",
"nested_filter": {
"range": {
"comments.date": {
"gte": "2014-10-01",
"lt": "2014-11-01"
}
}
}
}
}
}

Answer 2

This is what I ended up with (With the help from #ydrall - thank you very much! :>):
The query part (for us is works just fine without this first part):
"query": {
"bool": {
"must": [
{
"nested": {
"path": "groups",
"query": {
"bool": {
"must": [
{
"match": {
"groups.parentGroupId": 3
}
}
]
}
}
}
}
]
}
}
The sorting part of the query:
"sort": [
{
"groups.peerOrder": {
"order": "asc",
"nested_path": "groups",
"nested_filter": {
"match": {
"groups.parentGroupId": 3
}
}
}
}
Index mappings:
"mappings": {
"productmodel": {
"properties": {
"groups": {
"type": "nested",
"properties": {
"id": {
"type": "integer"
},
"parentGroupId": {
"type": "integer"
},
"peerOrder": {
"type": "integer"
}
}
}
}
}
}