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       mongoc_collection_aggregate - mongoc_collection_aggregate()


          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)


       · collection: A mongoc_collection_t.

       · flags: A mongoc_query_flags_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:  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

       · 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

       · batchSize: To specify the number of documents to return in each batch of a response from
         the server, include an int "batchSize" field.

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


       This function creates a cursor  which  sends  the  aggregate  command  on  the  underlying
       collection  upon  the first call to mongoc_cursor_next(). 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_prefsoptsoptsopts      │
                     │Transaction      │ Transaction  │ Transaction   │           │
                     │collectioncollectioncollection    │           │

       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.  The  user  must  call
       mongoc_cursor_next() on the returned mongoc_cursor_t to execute the aggregation pipeline.

          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;


       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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