find the mongo db unique documents

Question

I have a collection called location in mongodb. It has fields location:

 (_id,country,region,city,doctype,name)

There may be multiple documents( records ) with same name. I want to find all documents and select fields

_id,doctype,and name

with unique name. I am new to mongo db and please help me.

Answer 1

尝试db.location.distinct( 'name' );

Answer 2

Do a map reduce to count the name that are multiple:

db.collection.mapReduce(
    function(){emit(this.name);},
    function(key,values){
        sum=0;
        for(var i=0;i<values.length;i++){
           sum+=values[i];
        }
        return sum;
    },
    {out:"count_per_name"}
);

This will create a collection named "count_per_name". Query that to find the names with count bigger than 1.

Answer 3

You want to use the .aggregate function with the $group operator:

db.collection.aggregate([
    { "$group": {
        "_id": {
            "_id": "$_id",
            "doctype": " $doctype",
            "name": "$name"
        }
    }}
])

This is kind of the equivalent of the SQL "GROUP" operator. See the SQL TO Aggregation mapping in the MongoDB manual for more examples.