CouchDB calculate sums and averages of group - mapreduce

Question

I have bunch of documents like this:
{
"schema": "property",
"schema_version": 1,
"name": "test foobar"
"account": "1969360",
"web_data": {
"conversion_rate": 1.49,
"average_order_value": 123,
"visitor_count": 25000,
"visits_count": 35000,
"revenue": 50000
}
}
There could be multiple documents with same account.
Now I should create MapReduce to calculate average of web_data.conversion_rate and web_data.average_order_value and sum of web_data.visitor_count, web_data.visits_count and web_data.revenue for each document which has same account.
Currently my mapping function emits array like this:
['10449266', 'test foobar']
where first item is account and second name.
Emitted value is just web_data.
I would like to have output like this:
GET /list_properties?group_level=1
{
rows: [
{
key: [
"10449266"
],
value: {
"property_count": 2,
"conversion_rate": 1.8625,
"average_order_value": 153.75,
"visitor_count": 31250,
"visits_count": 43750,
"revenue": 62500
}
},
{
key: [
"66294401"
],
value: { ... }
},
...
}
How could I achieve this?
Pretty complex pattern, huh?
I've tried so many reduce functions to do this, I'm totally exhausted now. This is one I tried to get started:
function (keys, values, rereduce) {
return values.reduce(function(a, b){
return a + b.web_data.revenue;
});
}
That doesn't work as it returns object instead of number. And I have no idea why. It sums up numbers and returns object?
EDIT:
I updated reduce function to this:
function (keys, values, rereduce) {
if(rereduce){
return values;
}else{
var revenue = values.reduce(function(a, b){
return a + b.web_data.revenue;
}, 0);
return {
revenue : revenue
};
}
}
Now I get response like this:
{
"key": [
"2035864"
],
"value": [
[
{
"revenue": 0
},
{
"revenue": 66321.2
},
{
"revenue": 5319.35
}
],
[
{
"revenue": 0
}
],
[
{
"revenue": 45432.02
}
],
[
{
"revenue": 185732.78
}
]
]
}
Now if I change return values to return values[0] in rereduce-part, I get response like:
{
"key": [
"2035864"
],
"value": {
"revenue": 0
}
}
NONSENSE AGAIN!
Everything I want is just:
{
"key": [
"2035864"
],
"value": {
"revenue": 302805.35
}
}
JavaScript allows to do this so easily in browser but I don't understand why CouchDB wants to do things as weirdly as this.