mongoc_cursor_t *
mongoc_collection_aggregate (mongoc_collection_t *collection,
                             mongoc_query_flags_t flags,
                             const bson_t *pipeline,
                             const bson_t *opts,
                             const mongoc_read_prefs_t *read_prefs)



This function shall execute an aggregation query on the underlying ‘collection’. The bson ‘pipeline’ is not validated, simply passed along as appropriate to the server. As such, compatibility and errors should be validated in the appropriate server documentation.

In the case of older server versions, < v2.5, the returned cursor is a synthetic iterator over the result set. This provides a limitation insofar as returned documents can be no larger than 16MB. When connecting to newer servers this limitation doesn’t exist. The specific test is for wire_version > 0.

For more information on building MongoDB pipelines, see MongoDB Aggregation Command on the MongoDB website.


The pipeline parameter should contain a field named pipeline containing a BSON array of pipeline stages.

To target a specific server, include an integer “serverId” field in opts with an id obtained first by calling mongoc_client_select_server(), then mongoc_server_description_id() on its return value.

The mongoc_read_concern_t and the mongoc_write_concern_t specified on the mongoc_collection_t will be used, if any.


This function returns a newly allocated mongoc_cursor_t that should be freed with mongoc_cursor_destroy() when no longer in use. The returned mongoc_cursor_t is never NULL; if the parameters are invalid, the bson_error_t in the mongoc_cursor_t is filled out, and the mongoc_cursor_t is returned before the server is selected.


Failure to handle the result of this function is a programming error.


#include <bcon.h>
#include <mongoc.h>

static mongoc_cursor_t *
pipeline_query (mongoc_collection_t *collection)
   mongoc_cursor_t *cursor;
   bson_t *pipeline;

   pipeline = BCON_NEW ("pipeline",
                        BCON_UTF8 ("A"),
                        BCON_BOOL (false),

   cursor = mongoc_collection_aggregate (
      collection, MONGOC_QUERY_NONE, pipeline, NULL, NULL, NULL);

   bson_destroy (pipeline);

   return cursor;

Other Parameters

When using $out, the pipeline stage that writes, the write_concern field of the mongoc_cursor_t will be set to the mongoc_write_concern_t parameter, if it is valid, and applied to the write command when mongoc_cursor_next() is called. Pass any other parameters to the aggregate command, besides pipeline, as fields in opts:

mongoc_write_concern_t *write_concern = mongoc_write_concern_new ();
mongoc_write_concern_set_w (write_concern, 3);

pipeline =
   BCON_NEW ("pipeline", "[", "{", "$out", BCON_UTF8 ("collection2"), "}", "]");

opts = BCON_NEW ("bypassDocumentValidation", BCON_BOOL (true));
mongoc_write_concern_append (write_concern, opts);

cursor = mongoc_collection_aggregate (
   collection1, MONGOC_QUERY_NONE, pipeline, opts, NULL);