mongoc_collection_find - Man Page

mongoc_cursor_t *
mongoc_collection_find (mongoc_collection_t *collection,
                        mongoc_query_flags_t flags,
                        uint32_t skip,
                        uint32_t limit,
                        uint32_t batch_size,
                        const bson_t *query,
                        const bson_t *fields,
                        const mongoc_read_prefs_t *read_prefs);

• collection: A mongoc_collection_t.
• flags: A mongoc_query_flags_t.
• skip: A uint32_t of number of documents to skip or 0.
• limit: A uint32_t of max number of documents to return or 0.
• batch_size: A uint32_t containing batch size of document result sets or 0 for default. Default is 100.
• query: A bson_t containing the query and options to execute.
• fields: A bson_t containing fields to return or NULL.
• read_prefs: A mongoc_read_prefs_t or NULL for default read preferences.

This function shall execute a query on the underlying collection.

If no options are necessary, query can simply contain a query such as {a:1}. If you would like to specify options such as a sort order, the query must be placed inside of {"$query": {}}. See the example below for how to properly specify additional options to query.

This function is considered a retryable read operation. 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.

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:

Print All Documents in a Collection

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

static void
print_all_documents (mongoc_collection_t *collection)
{
   mongoc_cursor_t *cursor;
   bson_error_t error;
   const bson_t *doc;
   char *str;
   bson_t *query;

   query = BCON_NEW ("$query",
                     "{",
                     "foo",
                     BCON_INT32 (1),
                     "}",
                     "$orderby",
                     "{",
                     "bar",
                     BCON_INT32 (-1),
                     "}");
   cursor = mongoc_collection_find (
      collection, MONGOC_QUERY_NONE, 0, 0, 0, query, NULL, NULL);

   while (mongoc_cursor_next (cursor, &doc)) {
      str = bson_as_canonical_extended_json (doc, NULL);
      printf ("%s\n", str);
      bson_free (str);
   }

   if (mongoc_cursor_error (cursor, &error)) {
      fprintf (stderr, "An error occurred: %s\n", error.message);
   }

   mongoc_cursor_destroy (cursor);
   bson_destroy (query);
}

Queries have historically been sent as OP_QUERY wire protocol messages, but beginning in MongoDB 3.2 queries use the "find" command instead.

The driver automatically converts queries to the new "find" command syntax if needed, so this change is typically invisible to C Driver users. However, an application written exclusively for MongoDB 3.2 and later can choose to use the new syntax directly instead of relying on the driver to convert from the old syntax:

/* MongoDB 3.2+ "find" command syntax */
query = BCON_NEW ("filter",
                  "{",
                  "foo",
                  BCON_INT32 (1),
                  "}",
                  "sort",
                  "{",
                  "bar",
                  BCON_INT32 (-1),
                  "}");
cursor = mongoc_collection_find (
   collection, MONGOC_QUERY_NONE, 0, 0, 0, query, NULL, NULL);

The "find" command takes different options from the traditional OP_QUERY message.

Most applications should use the OP_QUERY syntax, with "$query", "$orderby", and so on, and rely on the driver to convert to the new syntax if needed.

SEE ALSO:

The "find" command in the MongoDB Manual.

With MongoDB before 3.2, a query with option $explain: true returns information about the query plan, instead of the query results. Beginning in MongoDB 3.2, there is a separate "explain" command. The driver will not convert "$explain" queries to "explain" commands, you must call the "explain" command explicitly:

/* MongoDB 3.2+, "explain" command syntax */
command = BCON_NEW ("explain",
                    "{",
                    "find",
                    BCON_UTF8 ("collection_name"),
                    "filter",
                    "{",
                    "foo",
                    BCON_INT32 (1),
                    "}",
                    "}");
mongoc_collection_command_simple (collection, command, NULL, &reply, &error);

SEE ALSO:

The "explain" command in the MongoDB Manual.

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