---
name: "mongoc_collection_aggregate"
section: "3"
platform: "linux"
package: "libmongoc-doc"
version: "1.30.4-1+deb13u3"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/mongoc_collection_aggregate"
---

# mongoc\_collection\_aggregate(3)

## 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);
```

## PARAMETERS

- •

  **collection**: A *mongoc\_collection\_t*.

- •

  **flags**: A *mongoc\_query\_flags\_t*. Not all flag values apply. In particular, setting **MONGOC\_QUERY\_EXHAUST** results in an error.

- •

  **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 **collection**, 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*.

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.

## DESCRIPTION

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\_prefs** | **opts** | **opts** | **opts** |
| Transaction | Transaction | Transaction |  |
| **collection** | **collection** | **collection** |  |

*See the example for transactions* and for *the "distinct" command with opts*.

## 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 *
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);
```

## AUTHOR

MongoDB, Inc

## COPYRIGHT

2009-present, MongoDB, Inc.