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;
}
来源:oschina
链接:https://my.oschina.net/u/4400642/blog/4291136