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

Read preferences, read and write concern, and collation can be overridden by various sources. The highest-priority sources for these options are listed first in the following table. In a transaction, read concern and write concern are prohibited in opts and the read preference must be primary or NULL. 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.

Read Preferences Read Concern Write Concern Collation
read_prefs opts opts opts
Transaction Transaction Transaction  
collection collection collection  

See the example for transactions and for the “distinct” command with opts.


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 <bson/bson.h>
#include <mongoc/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);