MongoDB count distinct value?

Question

Here below show my code. I have to calculate the how many times distinct value repeated. Here i have store distinct value in \"results\".I used collection.count() to calcula

Answer 1

While .distinct() works well for just obtaining the distinct values for a field, in order to actually get the counts of occurrences, this is better suited to the aggregation framework:

Collection.aggregate([
    { "$group": {
        "_id": "$field",
        "count": { "$sum": 1 }
    }}
],function(err,result) {

});

Also the .distinct() method does "abstract" from where the specified "distinct" field is actually within an array. In this case you need to call $unwind first to process the array elements here:

Collection.aggregate([
    { "$unwind": "$array" },
    { "$group": {
        "_id": "$array.field",
        "count": { "$sum": 1 }
    }}
],function(err,result) {

});

So the main work is basically done in the $group by "grouping" on the field values, which means the same thing as "distinct". The $sum is a grouping operator which in this case just adds up 1 for each occurrence of that value in the field for that collection.

Answer 2

To get occurrences of distinct values of a field 'field1' on a collection 'col1' and write to a separate collection 'distinctCount'. Also allow to use disk space in case the collection is huge.

db.col1.aggregate(
          [{$group: {
              _id: "$field1",
              count: { $sum : 1 }
            }}, {
            $group: {
              _id: "$_id",
              count: { $sum : "$count" }
            }},{
              $out: "distinctCount"
            }],
         {allowDiskUse:true}
)