display calculated data with CouchDB and PouchDB - view

Question

I'm trying to understand how to return calculated data on docs using CouchDB and PouchDB.
Say I have two types of docs on my CouchDB: Blocks and Reports.
Reports consists of: report_id, block_id and date.
Block consists of: block_id and name.
I'd like to calculate for each block it's last report_id (the id of the most recent report), and return it with block's doc.
Is there a way to achieve that?
I'm assuming that a View of some type will do the trick but I can't figure it out.

Answer 1

You can do this with map/reduce functions in CouchDB.
Let's say you have those documents :
{
"_id": "report_1",
"type": "report",
"block_id": "block_1",
"date": "1500325245"
}
{
"_id": "report_2",
"type": "report",
"block_id": "block_1",
"date": "1153170045"
}
You would like to get the reports with the highest timestamp (in this case, repot_1).
We start by creating a map function that will map the documents with the bloc_id as the key and the timestamp+ report id as the value for reduce function.
Map :
function (doc) {
if(doc.type == "report")
emit(doc.block_id,{date:doc.created,report:doc._id});
}
Then, we will create a reduce function. When rereduce is false, we will simply return the values. When rereduce is true, we will find the maximum timestamp and return the report id associated to it
Reduce function :
function (keys, values, rereduce) {
if (rereduce) {
var max = 0;
var maxReportId = -1;
for (var i = 0; i < values.length; i++) {
var val = values[i][0];
if (parseInt(val.date) > max) {
max = val.date;
maxReportId = val.report;
}
}
//We return the report id of the most recent report.
return maxReportId;
} else
return values;
}