Count Distinct Values via aggregation framework

Q: Is it possible to count distinct values of a field in mongodb?

A: Yes! This can be done via the aggregation framework in mongo. This takes two group commands; the first groups by all the distinct values, and the second does a count of them all.

pipeline = [ 
    { $group: { _id: "$myNonUniqueFieldId"}  },
    { $group: { _id: 1, count: { $sum: 1 } } }
];

db.runCommand( 
    {
    "aggregate": "collection" , 
    "pipeline": pipeline
    }
);