Compute difference between field and aggregated field - elasticsearch

Question

I have to run complex aggregation and one of its steps is computing sum of sold_qty field, and then I need to subtract this sum with non aggregated field all_qty. My data looks like:
{item_id: XXX, sold_qty: 1, all_qty: 20, price: 100 }
{item_id: XXX, sold_qty: 3, all_qty: 20, price: 100 }
{item_id: YYY, sold_qty: 1, all_qty: 20, price: 80 }
These are transactions from offer. The all_qty and price fields are redundant - express single values from other structure - offers and just duplicated in all transactions from single offer (identified by item_id).
In the terms of SQL what I need is:
SELECT (all_qty - sum(sold_qty)) * price GROUP BY item_id
What I've done is aggregation
'{
"query": {"term": {"seller": 9059247}},
"size": 0,
"aggs": {
"group_by_offer": {
"terms": { "field": "item_id", size: 0},
"aggs": { "sold_sum": {"sum": {"field": "sold_qty"}}}
}
}
}'
But I don't know what to do next to achieve my goal.

Answer 1

Since you are already storing redundant fields, if I were you, I would also store the result of all_price = all_qty * price and sold_price = sold_qty * price. It's is not mandatory but it will be faster at execution time than executing scripts to make the same computation.
{item_id: XXX, sold_qty: 1, sold_price: 20, all_qty: 20, price: 100, all_price: 2000 }
{item_id: XXX, sold_qty: 3, sold_price: 300, all_qty: 20, price: 100, all_price: 2000 }
{item_id: YYY, sold_qty: 1, sold_price: 80, all_qty: 20, price: 80, all_price: 1600 }
All you'd have to do next is to sum sold_price and average all_price and simply get the difference between both using a bucket_script pipeline aggregation:
{
"query": {
"term": {
"seller": 9059247
}
},
"size": 0,
"aggs": {
"group_by_offer": {
"terms": {
"field": "item_id",
"size": 0
},
"aggs": {
"sold_sum": {
"sum": {
"field": "sold_price"
}
},
"all_sum": {
"avg": {
"field": "all_price"
}
},
"diff": {
"bucket_script": {
"buckets_path": {
"sold": "sold_sum",
"all": "all_sum"
},
"script": "params.all - params.sold"
}
}
}
}
}
}