mongoc: Do not get $unwind to work with mongoDB c driver -

i have documents using array of subdocuments "c".

   {       "_id" : 1,       "c" : [         {           "p" : {             "name" : "simplegroup"           }         },         {           "p" : {             "name" : "simplegroup2"           }         }       ]     } 

running

db.collection.aggregate([ { "$unwind" : "$c" }, { "$project" : { "name" : "$c.p.name" } } ]) 

works fine in command line. if build same pipeline using mongoc driver looks $unwind entry ignored.

#include <bson.h> #include <mongoc.h>  bson_t pipeline; bson_init(&pipeline); bson_t pipearr, pipeelem1, pipeelem2; bson_append_array_begin(&pipeline, "pipeline", -1, &pipearr);  bson_append_document_begin(&pipearr, "0", -1, &pipeelem1); bson_append_utf8(&pipeelem1, "$unwind", -1, "$c", -1); bson_append_document_end(&pipearr, &pipeelem1);  bson_append_document_begin(&pipearr, "0", -1, &pipeelem1); bson_append_document_begin(&pipeelem1, "$project", -1, &pipeelem2); bson_append_utf8(&pipeelem2, "name", -1, "$c.p.name", -1); bson_append_document_end(&pipeelem1, &pipeelem2); bson_append_document_end(&pipearr, &pipeelem1);  bson_append_array_end(&pipeline, &pipearr);  cursor = mongoc_collection_aggregate (coll, mongoc_query_none, &pipeline, nullptr, nullptr); 

what done wrong?

expected result:

{ "_id" : 1, "name" : "simplegroup" } { "_id" : 1, "name" : "simplegroup2" } 

retrieved result:

{ "_id" : 1, "name" : [ "simplegroup", "simplegroup2" ] } 

---

additional

i tried bcon_new , worked. how $unwind correctly inserted using bson_append_??? .

bson_t *pipeline;  pipeline = bcon_new ("pipeline", "[",   "{", "$unwind", "$c", "}",   "{", "$project", "{", "name", "$c.p.name", "}", "}",   "]"); 

the problem creation of pipeline array. each array element has key inserted "0" causes overwrite.

bson_t pipeline; bson_init(&pipeline); bson_t pipearr, pipeelem1, pipeelem2; bson_append_array_begin(&pipeline, "pipeline", -1, &pipearr);  bson_append_document_begin(&pipearr, "0", -1, &pipeelem1); bson_append_utf8(&pipeelem1, "$unwind", -1, "$c", -1); bson_append_document_end(&pipearr, &pipeelem1);  bson_append_document_begin(&pipearr, "1", -1, &pipeelem1); bson_append_document_begin(&pipeelem1, "$project", -1, &pipeelem2); bson_append_utf8(&pipeelem2, "name", -1, "$c.p.name", -1); bson_append_document_end(&pipeelem1, &pipeelem2); bson_append_document_end(&pipearr, &pipeelem1);  bson_append_array_end(&pipeline, &pipearr); 

i missed aspect of bson_append_document_begin after bson_append_array_begin.