mongoc_database_aggregate()

Synopsis

mongoc_cursor_t *
mongoc_database_aggregate (mongoc_database_t *database,
                           const bson_t *pipeline,
                           const bson_t *opts,
                           const mongoc_read_prefs_t *read_prefs)
   BSON_GNUC_WARN_UNUSED_RESULT;

Parameters

• database: A mongoc_database_t.

• pipeline: A bson_t, either a BSON array or a BSON document containing an array field named “pipeline”.

• 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: 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: First, construct a mongoc_client_session_t with mongoc_client_start_session(). You can begin a transaction with mongoc_client_session_start_transaction(), optionally with a mongoc_transaction_opt_t that overrides the options inherited from database, 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.

• batchSize: An int32 representing number of documents requested to be returned on each call to mongoc_cursor_next()

• let: A BSON document consisting of any number of parameter names, each followed by definitions of constants in the MQL Aggregate Expression language.

• comment: A bson_value_t specifying the comment to attach to this command. The comment will appear in log messages, profiler output, and currentOp output. Only string values are supported prior to MongoDB 4.4.

• hint: A document or string that specifies the index to use to support the query predicate.

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

Description

This function creates a cursor which sends the aggregate command on the underlying database upon the first call to mongoc_cursor_next(). For more information on building aggregation pipelines, see the MongoDB Manual entry on the aggregate command. Note that the pipeline must start with a compatible stage that does not require an underlying collection (e.g. “$currentOp”, “$listLocalSessions”).

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 database.

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

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

This function is considered a retryable read operation unless the pipeline contains a write stage like $out or $merge. Upon a transient error (a network error, errors due to replica set failover, etc.) the operation is safely retried once. If retryreads is false in the URI (see mongoc_uri_t) the retry behavior does not apply.

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, even on error. The user must call mongoc_cursor_next() on the returned mongoc_cursor_t to execute the initial command.

Cursor errors can be checked with mongoc_cursor_error_document(). It always fills out the bson_error_t if an error occurred, and optionally includes a server reply document if the error occurred server-side.

Warning

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

Example

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

static mongoc_cursor_t *
current_op_query (mongoc_client_t *client)
{
   mongoc_cursor_t *cursor;
   mongoc_database_t *database;
   bson_t *pipeline;

   pipeline = BCON_NEW ("pipeline",
                        "[",
                        "{",
                        "$currentOp",
                        "{",
                        "}",
                        "}",
                        "]");

   /* $currentOp must be run on the admin database */
   database = mongoc_client_get_database (client, "admin");

   cursor = mongoc_database_aggregate (
      database, pipeline, NULL, NULL);

   bson_destroy (pipeline);
   mongoc_database_destroy (database);

   return cursor;
}