Aggregate a set of unique values from all documents in CouchDB - couchdb

Question

I'm trying to create a view in CouchDB that returns groups of unique values. E.g., a list of all unique brands and categories.
map function
function (doc) {
emit("brands", [doc.brand]);
emit("categories", [doc.category]);
}
reduce function
function (keys, values, rereduce) {
return values.reduce(function(acc, value) {
if (acc.indexOf(value[0]) === -1) {
return acc.concat(value);
}
return acc;
});
}
then I call that view with group=true, group_level=2. The grouping is correct, but the values aren't unique. The value is an array containing duplicates.
What I'm trying to achieve is basically having the key be the group name, e.g., brands, and the value be the aggregated unique values, e.g., ["Brand A", "Brand B"].
Given the following documents
[
{
"_id": "1",
"brand": "Brand A",
"category": "Category A",
"colors": [
"Red",
"White"
]
},
{
"_id": "2",
"brand": "Brand B",
"category": "Category B",
"colors": [
"Blue",
"White"
]
},
{
"_id": "3",
"brand": "Brand A",
"category": "Category B",
"colors": [
"Green",
"Red"
]
}
]
When I query then view in CouchDB, I'd like to get the following result back
{
"brands": ["Brand A", "Brand B"],
"categories": ["Category A", "Category B"],
"colors": ["Red", "White", "Blue", "Green"]
}
Note: The result above is just a demonstration of what I expect the view to return. It does not have to be structured as such (not even sure it's possible).

Answer 1

I'm going to answer this myself.
First, we want to define a map function that emit the group name as the key and the value wrapped in an array (making rereduce easier).
map function
function (doc) {
emit("brands", [doc.brand]);
emit("categories", [doc.category]);
doc.colors.forEach(function(color) {
emit("colors", [color]);
})
}
The we define a custom reduce function
function (keys, values, rereduce) {
return values.reduce(function(acc, value) {
value.forEach(function(v) {
if (acc.indexOf(v) === -1) {
return acc.push(v);
}
});
return acc;
});
}
Now, calling the view with group=true and group_level=1 will yield the following result:
+------------+-----------------------------------+
| key | value |
+------------+-----------------------------------+
| brands | ["Brand A", "Brand B"] |
| categories | ["Category A", "Category B"] |
| colors | ["Red", "White", "Blue", "Green"] |
+------------+-----------------------------------+