MongoDB grouping based on intervals - mongodb

Question

I would like to group my data based on number intervals in measurements. Can I do this with the aggregation framework, or with some map-reduce function?
I would like to group by color and whether the size is larger or smaller than 5. I would also want to add e.g. "medium" for sizes between 3 and 5.
I can group by size and color, but then each different size will have its own object.
I know this can be done by checking each different object's size by db.collection.find(), and then adding them according to my specifications, but that would be very slow.
Example:
Objects:
{
color: "red",
size: 2
}
{
color: "red",
size: 4
}
{
color: "blue",
size: 2
}
{
color: "blue",
size: 1
}
{
color: "blue",
size: 7
}
Output:
{
_id: {
color: "red",
size: "small"
}
total size: 6
}
{
_id: {
color: "red",
size: "large"
}
total size: 0
}
{
_id: {
color: "blue",
size: small
}
total size: 3
}
{
_id: {
color: "blue",
size: "large"
}
total size: 7
}

Answer 1

This is easy using $cond:
db.collection.aggregate([
{ "$group": {
"_id": {
"color": "$color",
"size": {
"$cond": [
{ "$lt": [ "$size", 3 ] },
"small",
{ "$cond": [
{ "$lt": [ "$size", 6 ] },
"medium",
"large"
]}
]
}
},
"total_size": { "$sum": "$size" }
}}
])
So just conditionally select the value in the grouping key based on the current value in the document and count.