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)


opts may be NULL or a BSON document with additional command options:

For a list of all options, see the MongoDB Manual entry on the aggregate command.


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.

For more information on building MongoDB pipelines, see the MongoDB Manual entry on the aggregate command.


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

Read concern is applied from opts or else from collection. Collation is applied from opts (see example for the “distinct” command with opts). Read preferences are applied from read_prefs or else from collection. Write concern is applied from opts, or if opts has no write concern and the aggregation pipeline includes “$out”, the write concern is applied from collection. The write concern is omitted for MongoDB before 3.4.


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);

   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);