Solr: group by child document field and include parent fields - solr

Question

Is there a way to group by a child document field and include the parent fields to the result?
Imagine you have
[
{
"id": "p1",
"name": "parent 1",
"_childDocuments_": [
{
"id": "p1c1",
"name": "child1_of_parent1",
"color": "red"
},
{
"id": "p1c2",
"name": "child2_of_parent1",
"color": "yellow"
}
]
},
{
"id": "p2",
"name": "parent 2",
"_childDocuments_": [
{
"id": "p2c1",
"name": "child1_of_parent2",
"color": "yellow"
}
]
}
]
in a collection.
Now a query
/select?group=true&group.field=color&group.limit=10
returns
{
"responseHeader":{
"params":{
"group.limit":"10",
"group.field":"color",
"group":"true"
}
},
"grouped":{
"color":{
"matches":3,
"groups":[
{
"groupValue":"red",
"doclist":{"numFound":1,"docs":[
{
"id":"p1c1",
"name":"child1_of_parent1"
}
]
}
},
{
"groupValue":"yellow",
"doclist":{"numFound":2,"docs":[
{
"id":"p1c2",
"name":"child2_of_parent1"
},
{
"id":"p2c1",
"name":"child1_of_parent2"
}
]
}
}
]
}
}
}
But I need a result that contains their parent fields as well, something like
{
"responseHeader":{
"params":{
"group.limit":"10",
"group.field":"color",
"group":"true"
}
},
"grouped":{
"color":{
"matches":3,
"groups":[
{
"groupValue":"red",
"doclist":{"numFound":1,"docs":[
{
"id":"p1c1",
"name":"child1_of_parent1",
"parent":{
"id": "p1",
"name": "parent 1",
}
}
]
}
},
{
"groupValue":"yellow",
"doclist":{"numFound":2,"docs":[
{
"id":"p1c2",
"name":"child2_of_parent1",
"parent":{
"id": "p1",
"name": "parent 1",
}
},
{
"id":"p2c1",
"name":"child1_of_parent2",
"parent":{
"id": "p2",
"name": "parent 2",
}
}
]
}
}
]
}
}
}
I'm coming from relational databases, where this can easily be done. Hopefully there's a way in solr as well. I'm using solr 8.7.0

Answer 1

One solution I found is subquery. It satisfies the requirements, but the performance is what is to expect when you transfer a relational "join" to a document db.
It will definitely be a better idea to rework the datamodel to a flat structure.
Before I had to add a field in the child doc for the parent id (the default "root" doesn't work):
[
{
"id": "p1",
"name": "parent 1",
"_childDocuments_": [
{
"id": "p1c1",
"name": "child1_of_parent1",
"color": "red",
"parent_id": "p1"
},
{
"id": "p1c2",
"name": "child2_of_parent1",
"color": "yellow",
"parent_id": "p1"
}
]
},
{
"id": "p2",
"name": "parent 2",
"_childDocuments_": [
{
"id": "p2c1",
"name": "child1_of_parent2",
"color": "yellow",
"parent_id": "p2"
}
]
}
]
Now I can query
/select?group=true&group.field=color&group.limit=10&fl=*%2Cparent%3A%5Bsubquery%5D&parent.q=%7B%21terms+f%3Did+v%3D%24row.parent_id%7D
and it returns
{
"responseHeader":{
"params":{
"group.limit":"10",
"group.field":"color",
"group":"true",
"fl":"*,parent:[subquery]",
"parent.q":"{!terms f=id v=$row.parent_id}",
}
},
"grouped":{
"color":{
"matches":3,
"groups":[
{
"groupValue":"red",
"doclist":{"numFound":1,"docs":[
{
"id":"p1c1",
"name":"child1_of_parent1",
"parent":{"numFound":1,"start":0,"numFoundExact":true,"docs":[
"id": "p1",
"name": "parent 1",
]}
}
]
}
},
{
"groupValue":"yellow",
"doclist":{"numFound":2,"docs":[
{
"id":"p1c2",
"name":"child2_of_parent1",
"parent":{"numFound":1,"start":0,"numFoundExact":true,"docs":[
"id": "p1",
"name": "parent 1",
]}
},
{
"id":"p2c1",
"name":"child1_of_parent2",
"parent":{"numFound":1,"start":0,"numFoundExact":true,"docs":[
"id": "p2",
"name": "parent 2",
]}
}
]
}
}
]
}
}
}
Feel free to comment if that reminds you at a better idea.