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- mongoc_collection_t

# 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 *opts,
                                 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.
- `opts`: A `bson_t` containing options for the command, or `NULL`.
- `read_prefs`: A mongoc_read_prefs_t or `NULL`.

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

- `readConcern`: Construct a mongoc_read_concern_t and use mongoc_read_concern_append() to add the read concern to `opts`. See the example code for mongoc_client_read_command_with_opts(). Read concern requires MongoDB 3.2 or later, otherwise an error is returned.
- `writeConcern`: For aggregations that include “$out”, you can construct a mongoc_write_concern_t and use mongoc_write_concern_append() to add the write concern to `opts`. See the example code for mongoc_client_write_command_with_opts().
- `sessionId`: Construct a mongoc_client_session_t with mongoc_client_start_session() and use mongoc_client_session_append() to add the session to `opts`. See the example code for mongoc_client_session_t.
- `bypassDocumentValidation`: Set to `true` to skip server-side schema validation of the provided BSON documents.
- `collation`: Configure textual comparisons. See Setting Collation Order, and the MongoDB Manual entry on Collation. Collation requires MongoDB 3.2 or later, otherwise an error is returned.
- `serverId`: To target a specific server, include an int32 “serverId” field. Obtain the id by calling mongoc_client_select_server(), then mongoc_server_description_id() on its return value.

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

## 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.

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

Note

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.

## Returns

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.

Warning

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

## 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

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);

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