Solr faceting based on function result - solr

Question

I'm trying to perform faceting based on a dynamic value. Basically I want identical behavior to the def function, but that doesn't seem to be available with faceting.
Consider these two "products":
{
"id":"product1",
"defaultPrice":19.99,
"overridePrice":14.99
},
{
"id":"product2",
"defaultPrice":49.99
}
I want to add that overridePrice is just an example. The actual field is a dynamic value that will depend on what context a search is performed in, and there may be many overridden prices, so I can't just derive price at index time.
For the response, I'm doing something like this for fl:
fl=price:def(overridePrice, defaultPrice) and using the same def function to perform sorting on price. This works fine.
So now I want to apply the same logic to facets. I've tried using json.facet, which seemed like it would work:
json.facet={
price: "def(overridePrice, defaultPrice)"
}
I've tried other variations as well, such as using field:def(overridePrice, defaultPrice) as well as field:price, but def doesn't seem to be an available function for faceting, and the price derived field is not available when faceting.
So the question: How can I perform faceting based on a default field like I'm doing for fl and sorting? Will this require a custom aggregation function, or is there a clever way I can do this without a custom function? It would be much more preferable to be able to do this with built-in Solr functionality.

Answer 1

I was able to do a hacky solution based on a tip in another question.
We can use two facets with a query to filter documents depending on if a field exists.
Example:
{
price_override: {
type: query,
q: "overridePrice:[* TO *]",
facet: {
price_override:{
type:terms,
field: overridePrice
}
}
},
price_standard: {
type: query,
q: "-overridePrice:[* TO *] AND defaultPrice:[* TO *]",
facet: {
price_standard: {
type: terms,
field: defaultPrice
}
}
}
}
Explanation:
price_override: {
type: query,
q: "overridePrice:[* TO *]"
This range query only selects documents that have an overridePrice field.
price_standard: {
type: query,
q: "-overridePrice:[* TO *] AND defaultPrice:[* TO *]"
-overridePrice:[* TO *] omits documents with the overridePrice field, and selects documents with a defaultPrice field.
And the facet response:
"facets":{
"count":2,
"price_override":{
"count":1,
"price_override":{
"buckets":[{
"val":14.99,
"count":1}]}},
"price_standard":{
"count":1,
"price_standard":{
"buckets":[{
"val":49.99,
"count":1}]}}}
This does require manually grouping price_override and price_standard into a single facet group, but the results are as expected. This could also pretty easily be tweaked into a ranged query, which is my use case.