Couchdb: filter and group in a single view

I have a Couchdb database with documents of the form: { Name, Timestamp, Value }

I have a view that shows a summary grouped by name with the sum of the values. This is straigh开发者_如何学Ct forward reduce function.

Now I want to filter the view to only take into account documents where the timestamp occured in a given range.

AFAIK this means I have to include the timestamp in the emitted key of the map function, eg. emit([doc.Timestamp, doc.Name], doc)

But as soon as I do that the reduce function no longer sees the rows grouped together to calculate the sum. If I put the name first I can group at level 1 only, but how to I filter at level 2?

Is there a way to do this?

---

I don't think this is possible with only one HTTP fetch and/or without additional logic in your own code.

If you emit([time, name]) you would be able to query startkey=[timeA]&endkey=[timeB]&group_level=2 to get items between timeA and timeB grouped where their timestamp and name were identical. You could then post-process this to add up whenever the names matched, but the initial result set might be larger than you want to handle.

An alternative would be to emit([name,time]). Then you could first query with group_level=1 to get a list of names [if your application doesn't already know what they'll be]. Then for each one of those you would query startkey=[nameN]&endkey=[nameN,{}]&group_level=2 to get the summary for each name.

(Note that in my query examples I've left the JSON start/end keys unencoded, so as to make them more human readable, but you'll need to apply your language's equivalent of JavaScript's encodeURIComponent on them in actual use.)

---

You can not make a view onto a view. You need to write another map-reduce view that has the filtering and makes the grouping in the end. Something like:

map:
function(doc) {
  if (doc.timestamp > start and doc.timestamp < end ) {
    emit(doc.name, doc.value);
  }
}

reduce:
function(key, values, rereduce) {
  return sum(values);
}

I suppose you can not store this view, and have to put it as an ad-hoc query in your application.