MongoDB select distinct and count - mongodb

Question

I have a product collection which looks like that:
products = [
{
"ref": "1",
"facets": [
{
"type":"category",
"val":"kitchen"
},
{
"type":"category",
"val":"bedroom"
},
{
"type":"material",
"val":"wood"
}
]
},
{
"ref": "2",
"facets": [
{
"type":"category",
"val":"kitchen"
},
{
"type":"category",
"val":"livingroom"
},
{
"type":"material",
"val":"plastic"
}
]
}
]
I would like to select and count the distinct categories and the number of products that have the category (Note that a product can have more than one category). Something like that:
[
{
"category": "kitchen",
"numberOfProducts": 2
},
{
"category": "bedroom",
"numberOfProducts": 1
},
{
"category": "livingroom",
"numberOfProducts": 1
}
]
And it would be better if I could get the same result for each different facet type, something like that:
[
{
"facetType": "category",
"distinctValues":
[
{
"val": "kitchen",
"numberOfProducts": 2
},
{
"val": "livingroom",
"numberOfProducts": 1
},
{
"val": "bedroom",
"numberOfProducts": 1
}
]
},
{
"facetType": "material",
"distinctValues":
[
{
"val": "wood",
"numberOfProducts": 1
},
{
"val": "plastic",
"numberOfProducts": 1
}
]
}
]
I am doing tests with distinct, aggregate and mapReduce. But can't achieve the results needed. Can anybody tell me the good way?
UPDATE:
With aggregate, this give me the different facet categories that a product have, but not the values nor the count of different values:
db.products.aggregate([
{$match:{'content.facets.type':'category'}},
{$group:{ _id: '$content.facets.type'} }
]).pretty();

Answer 1

The following aggregation pipeline will give you the desired result. In the first pipeline step, you need to do an $unwind operation on the facets array so that it's deconstructed to output a document for each element. After the $unwind stage is the first of the $group operations which groups the documents from the previous stream by category and type and calculates the number of products in each group using $sum. The next $group operation in the next pipeline stage then creates the array that holds the aggregated values by using $addToSet operator. The final pipeline stage is the $project operation which then transforms the document in the stream by modifying existing fields:
var pipeline = [
{ "$unwind": "$facets" },
{
"$group": {
"_id": {
"facetType": "$facets.type",
"value": "$facets.val"
},
"count": { "$sum": 1 }
}
},
{
"$group": {
"_id": "$_id.facetType",
"distinctValues": {
"$addToSet": {
"val": "$_id.value",
"numberOfProducts": "$count"
}
}
}
},
{
"$project": {
"_id": 0,
"facetType": "$_id",
"distinctValues": 1
}
}
];
db.product.aggregate(pipeline);
Output
/* 0 */
{
"result" : [
{
"distinctValues" : [
{
"val" : "kitchen",
"numberOfProducts" : 2
},
{
"val" : "bedroom",
"numberOfProducts" : 1
},
{
"val" : "livingroom",
"numberOfProducts" : 1
}
],
"facetType" : "category"
},
{
"distinctValues" : [
{
"val" : "wood",
"numberOfProducts" : 1
},
{
"val" : "plastic",
"numberOfProducts" : 1
}
],
"facetType" : "material"
}
],
"ok" : 1
}