# mongoc_collection_aggregate()

## Synopsis

    mongoc_cursor_t *
    mongoc_collection_aggregate (mongoc_collection_t       *collection,
                                 mongoc_query_flags_t       flags,
                                 const bson_t              *pipeline,
                                 const bson_t              *options,
                                 const mongoc_read_prefs_t *read_prefs)
       BSON_GNUC_WARN_UNUSED_RESULT;

## Parameters

collection | A mongoc_collection_t.
flags | A mongoc_query_flags_t.
pipeline | A bson_t containing the pipeline array.
options | A bson_t containing options for the pipeline, or NULL.
read_prefs | A mongoc_read_prefs_t or NULL.

## Description

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.

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

## Returns

This function returns a newly allocated mongoc_cursor_t that should be freed with mongoc_cursor_destroy() when no longer in use. NULL may be returned if parameters are invalid.

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

## Example

### Aggregation Example

    #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", "[",
                               "{", "$match", "{", "foo", BCON_UTF8 ("A"), "}", "}",
                               "{", "$match", "{", "bar", BCON_BOOL (false), "}", "}",
                            "]");

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

       bson_destroy (pipeline);

       return cursor;
    }

## Other Parameters

Pass any parameters to the aggregate command, besides pipeline, as fields in options:

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

    options = BCON_NEW ("bypassDocumentValidation", BCON_BOOL (true));
    cursor = mongoc_collection_aggregate (collection1,
                                          MONGOC_QUERY_NONE,
                                          pipeline,
                                          options,
                                          NULL);