Question

Documents:

{ "group" : "G1", "cat" : "Cat1", "desc": "Some description 1"}
{ "group" : "G1", "cat" : "Cat2", "desc": "Some description 2"}
{ "group" : "G1", "cat" : "Cat1", "desc": "Some description 3"}
{ "group" : "G1", "cat" : "Cat3", "desc": "Some description 4"}
{ "group" : "G1", "cat" : "Cat2", "desc": "Some description 4"}

Can someone help me, using Mongoose, how to find the records that have unique group and cat?

From the Mongoose API for distinct, I understand the that I can use just one field. But can the Model.distinct be used to find documents based on two fields?

Answer 1

I can’t give you a Mongoose specific example, and your question is a bit vague. The aggregation equivalent of “But can the Model.distinct be used to find documents based on two fields?” is:

db.test.aggregate( { $group: { _id: { group: "$group", cat: "$cat" } } } );

Which returns:

{
    "result" : [
        {
            "_id" : {
                "group" : "G1",
                "cat" : "Cat3"
            }
        },
        {
            "_id" : {
                "group" : "G1",
                "cat" : "Cat2"
            }
        },
        {
            "_id" : {
                "group" : "G1",
                "cat" : "Cat1"
            }
        }
    ],
    "ok" : 1
}

If you want to find group/cat combinations that only occur once, then you would use:

db.test.aggregate(
    { $group: {
        _id: { group: "$group", cat: "$cat" }, 
        c: { $sum: 1 }, 
        doc_ids: { $addToSet: "$_id" }
    } },
    { $match : { c: 1 } }
);

Which returns:

{
    "result" : [
        {
            "_id" : {
                "group" : "G1",
                "cat" : "Cat3"
            },
            "c" : 1,
            "doc_ids" : [
                ObjectId("5112699b472ac038675618f1")
            ]
        }
    ],
    "ok" : 1
}

From http://mongoosejs.com/docs/api.html#model_Model.aggregate I learn that you can use the aggregation framework in Mongoose like:

YourModel.aggregate(
    { $group: { _id: { group: "$group", cat: "$cat" } } },
    function(err, result) {
        console.log(result)
    }
)