MongoDB MapReduce 小例子

var map = function(){
    if (this.gscode == "ZTJB"){
        ymd = this.ymd;
        emit("maxymd", ymd);
    }
}
var reduce = function(key, values){
    var maxYmd = values[0];
    for (var i=1; i<values.length; i++){
        if (maxYmd < values[i]){
            maxYmd = values[i];
        }
    }
    return maxYmd;
}
db.getCollection('calcgsdataflash').mapReduce(
    map,
    reduce,
    {out:{inline:1}}
);

C API 小例子：

　　官方文档地址：http://mongoc.org/libmongoc/1.8.2/distinct-mapreduce.html

vector<string> vMaxDate;
const char *const MAPPER = "function(){"
	"Date = this.Date;"
	"emit('maxDate',Date)"
"}";

const char *const REDUCER = "function(key, values){"
	"var maxDate = values[0];"
	"for (var i=1; i<values.length; i++){"
		"if (maxDate < values[i]){"
			"maxDate = values[i];"
		"}"
	"}"
	"return maxDate;"
"}";

m_mongoDBMgr.MapReduce(MAPPER, REDUCER, vMaxDate);
for (vector<string>::iterator iter = vMaxDate.begin(); iter != vMaxDate.end(); iter++)
{
	Json::Reader reader;
	Json::Value value;
	if (reader.parse((*iter).c_str(), value))
	{
		iMaxDate = value["value"].asInt();
		TLOG_DEBUG("L2Dynamic iMaxDate:" << iMaxDate << endl);
	}
}//maxYmd

bool CMongoDBMgr::MapReduce(const char *const MAPPER, const char *const REDUCER, vector<string> &vData)
{
    TLOG_DEBUG("begin MapReduce" << endl);

    bson_t reply;
    bson_t *command;
    bson_error_t error;
    mongoc_cursor_t *cursor;
    const bson_t *doc;

    bool map_reduce_done = false;
    bool query_done = false;

    const char *out_collection_name = "outCollection";
    mongoc_collection_t *out_collection;

    bson_t find_query = BSON_INITIALIZER;

    //do MapReduce
    command = BCON_NEW (
        "mapReduce",
        BCON_UTF8 (m_sCollection.c_str()),
        "map",
        BCON_CODE (MAPPER),
        "reduce",
        BCON_CODE (REDUCER),
        "out",
        BCON_UTF8 (out_collection_name)
    );
    bool bRet = mongoc_database_command_simple (m_database, command, NULL, &reply, &error);
    map_reduce_done = true;
    if (!bRet)
    {
        TLOG_DEBUG("MapReduce failed:" << error.message << endl);
        goto cleanup;
        return false;
    }

    //do query
    out_collection = mongoc_database_get_collection (m_database, out_collection_name);

    cursor = mongoc_collection_find_with_opts (out_collection, &find_query, NULL, NULL);
    query_done = true;

    while (mongoc_cursor_next (cursor, &doc))
    {
        char *str;
        str = bson_as_json(doc, NULL);
        vData.push_back(str);
    }

    if (mongoc_cursor_error (cursor, &error))
    {
        TLOG_DEBUG("An error occurred:" << error.message << endl);
        goto cleanup;
        return false;
    }

cleanup:
    if (map_reduce_done)
    {
        bson_destroy (&reply);
        bson_destroy (command);
    }
    
    if (query_done)
    {
        mongoc_cursor_destroy(cursor);
        mongoc_collection_destroy (out_collection);
    }

    return true;
}