Solr faceting on a Query Function result - solr

Question

Is it possible to produce solr facets for a field which is the result of Query Function?
I have an index of products with a price field for each store they are available in:
{
"id" : "p1",
"name_s" : "Product 1",
"description_s" : "The first product",
"price_l1_d" : 19.99,
"price_l2_d" : 20.00,
"price_l3_d" : 20.99,
"price_l4_d" : 19.99,
"price_l5_d" : 25.00,
"price_l6_d" : 18.00
},
{
"id" : "p2",
"name_s" : "Product 2",
"description_s" : "The second product",
"price_l1_d" : 12.99,
"price_l2_d" : 15.00,
"price_l3_d" : 13.49,
"price_l4_d" : 14.00,
"price_l5_d" : 12.50,
"price_l6_d" : 16.00
}
and I need my query to return the cheapest price in the customer's 3 closest stores.
I know I can return this value using fl=min(price_l2_d, price_l4_d, price_l6_d) and I can even sort on this but is it possible to return a "Price" facet based on this value for each document? Ideally I'd like to be able to show all products whose minimum price (in my 3 stores) is between 0-5, 5-10, 10-15, 15-20 etc etc and filter on this.
I've tried using min(price_l2_d, price_l4_d, price_l6_d) as facet.field but I receive an undefined field error. Is there a better way?
I cannot produce this value at index time because the closest 3 stores could be any combination of three price fields (in this example there is 6 but thee are likely to be over 200)

Answer 1

While not THE solution, I have found A solution which should work. Unfortunately it's not possible to create a traditional facet for price ranges as you would with a single integer attribute, but a two-point slider is possible.
Using the JSON facet API (as suggested by a comment on the original question) and the following:
{
"max" : "max(min(price_l2_d, price_l4_d, price_l6_d))",
"min" : "min(min(price_l2_d, price_l4_d, price_l6_d))"
}
I can return the boundaries of the slider with the smallest minimum price at the three stores and the biggest minimum price.
The values on this slider can then be applied using the {!frange} function as follows:
fq={!frange l=0 u=20}min(price_l2_d, price_l4_d, price_l6_d)
where l is the lower bound and u is the upper bound
Hopefully this helps anyone else looking for an answer to this.