Question

I have a data. Let me show you the example of that

1
1
2
2
2
3

And, I’d like to get the number of each values using Group by. The result will be like below

value | count
1 | 2
2 | 3
3 | 1

Maybe, the group by query on Mongo Shell is like this

db.collection.group(
{ key : {value:true},
reduce: function(obj, prev) { prev.csum += 1; },
initial: { csum: 0 }
})

But, I have to convert that query to C code with mongoDB C API.
I tried to make some code using bson_append_bson like that.. but, failed..

What shoould I do?

Answer 1

I setup a Mongo 2.2.1 instance on my machine on port 12345. I created a test.data collection and populated it with

use test
db.data.insert({value:1})
db.data.insert({value:1})
db.data.insert({value:2})
db.data.insert({value:2})
db.data.insert({value:2})
db.data.insert({value:3})

per your sample data. Using the latest driver, I wrote the following sample code.

#include <stdio.h>
#include <mongo.h>

int main( char *argv[] ) {
    bson b, out;
    mongo conn;

    if( mongo_connect( &conn, "127.0.0.1", 12345 ) != MONGO_OK ) {
        switch( conn.err ) {
            case MONGO_CONN_NO_SOCKET:
                printf( "FAIL: Could not create a socket!\n" );
                break;
            case MONGO_CONN_FAIL:
                printf( "FAIL: Could not connect to mongod. Make sure it's listening at 127.0.0.1:27017.\n" );
                break;
            }

        exit( 1 );
    }

    bson_init( &b );
        bson_append_string( &b, "aggregate", "data" );
        bson_append_start_array( &b, "pipeline" );
            bson_append_start_object( &b, "0" );
                bson_append_start_object( &b, "$group" );
                    bson_append_string( &b, "_id", "$value" );
                    bson_append_start_object( &b, "count" );
                        bson_append_int( &b, "$sum", 1 );
                    bson_append_finish_object( &b );
                bson_append_finish_object( &b );
            bson_append_finish_object( &b );
        bson_append_finish_array( &b );
    bson_finish( &b );
    bson_print( &b );

    printf( "command result code: %d\n", mongo_run_command( &conn, "test", &b, &out ) );
    bson_print( &out );

    bson_destroy( &b );
    bson_destroy( &out );
    mongo_destroy( &conn );
    return( 0 );
}

Instead of the $group command, which doesn’t work in sharded environments, I used the Aggregation Framework (new since 2.1). The above C code is the equivalent to

use test
db.data.aggregate({$group:{_id:"$value",count:{$sum:1}}})

in the Mongo shell.