Common Tasks

Drivers for some other languages provide helper functions to perform certain common tasks. In the C Driver we must explicitly build commands to send to the server.

Setup

First we’ll write some code to insert sample data:

doc-common-insert.c

/* Don't try to compile this file on its own. It's meant to be #included
   by example code */

/* Insert some sample data */
bool
insert_data (mongoc_collection_t *collection)
{
   mongoc_bulk_operation_t *bulk;
   enum N { ndocs = 4 };
   bson_t *docs[ndocs];
   bson_error_t error;
   int i = 0;
   bool ret;

   bulk = mongoc_collection_create_bulk_operation_with_opts (collection, NULL);

   docs[0] = BCON_NEW ("x", BCON_DOUBLE (1.0), "tags", "[", "dog", "cat", "]");
   docs[1] = BCON_NEW ("x", BCON_DOUBLE (2.0), "tags", "[", "cat", "]");
   docs[2] = BCON_NEW ("x", BCON_DOUBLE (2.0), "tags", "[", "mouse", "cat", "dog", "]");
   docs[3] = BCON_NEW ("x", BCON_DOUBLE (3.0), "tags", "[", "]");

   for (i = 0; i < ndocs; i++) {
      mongoc_bulk_operation_insert (bulk, docs[i]);
      bson_destroy (docs[i]);
      docs[i] = NULL;
   }

   ret = mongoc_bulk_operation_execute (bulk, NULL, &error);

   if (!ret) {
      fprintf (stderr, "Error inserting data: %s\n", error.message);
   }

   mongoc_bulk_operation_destroy (bulk);
   return ret;
}

/* A helper which we'll use a lot later on */
void
print_res (const bson_t *reply)
{
   char *str;
   BSON_ASSERT (reply);
   str = bson_as_canonical_extended_json (reply, NULL);
   printf ("%s\n", str);
   bson_free (str);
}

“explain” Command

This is how to use the explain command in MongoDB 3.2+:

explain.c

bool
explain (mongoc_collection_t *collection)
{
   bson_t *command;
   bson_t reply;
   bson_error_t error;
   bool res;

   command =
      BCON_NEW ("explain", "{", "find", BCON_UTF8 (COLLECTION_NAME), "filter", "{", "x", BCON_INT32 (1), "}", "}");
   res = mongoc_collection_command_simple (collection, command, NULL, &reply, &error);
   if (!res) {
      fprintf (stderr, "Error with explain: %s\n", error.message);
      goto cleanup;
   }

   /* Do something with the reply */
   print_res (&reply);

cleanup:
   bson_destroy (&reply);
   bson_destroy (command);
   return res;
}

Running the Examples

common-operations.c

/*
 * Copyright 2016 MongoDB, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */


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


const char *COLLECTION_NAME = "things";

#include "../doc-common-insert.c"
#include "explain.c"


int
main (int argc, char *argv[])
{
   mongoc_database_t *database = NULL;
   mongoc_client_t *client = NULL;
   mongoc_collection_t *collection = NULL;
   mongoc_uri_t *uri = NULL;
   bson_error_t error;
   char *host_and_port;
   int res = 0;

   if (argc < 2 || argc > 3) {
      fprintf (stderr,
               "usage: %s MONGOD-1-CONNECTION-STRING "
               "[MONGOD-2-HOST-NAME:MONGOD-2-PORT]\n",
               argv[0]);
      fprintf (stderr,
               "MONGOD-1-CONNECTION-STRING can be "
               "of the following forms:\n");
      fprintf (stderr, "localhost\t\t\t\tlocal machine\n");
      fprintf (stderr, "localhost:27018\t\t\t\tlocal machine on port 27018\n");
      fprintf (stderr,
               "mongodb://user:pass@localhost:27017\t"
               "local machine on port 27017, and authenticate with username "
               "user and password pass\n");
      return EXIT_FAILURE;
   }

   mongoc_init ();

   if (strncmp (argv[1], "mongodb://", 10) == 0) {
      host_and_port = bson_strdup (argv[1]);
   } else {
      host_and_port = bson_strdup_printf ("mongodb://%s", argv[1]);
   }

   uri = mongoc_uri_new_with_error (host_and_port, &error);
   if (!uri) {
      fprintf (stderr,
               "failed to parse URI: %s\n"
               "error message:       %s\n",
               host_and_port,
               error.message);
      res = EXIT_FAILURE;
      goto cleanup;
   }

   client = mongoc_client_new_from_uri (uri);
   if (!client) {
      res = EXIT_FAILURE;
      goto cleanup;
   }

   mongoc_client_set_error_api (client, 2);
   database = mongoc_client_get_database (client, "test");
   collection = mongoc_database_get_collection (database, COLLECTION_NAME);

   printf ("Inserting data\n");
   if (!insert_data (collection)) {
      res = EXIT_FAILURE;
      goto cleanup;
   }

   printf ("explain\n");
   if (!explain (collection)) {
      res = EXIT_FAILURE;
      goto cleanup;
   }

cleanup:
   if (collection) {
      mongoc_collection_destroy (collection);
   }

   if (database) {
      mongoc_database_destroy (database);
   }

   if (client) {
      mongoc_client_destroy (client);
   }

   if (uri) {
      mongoc_uri_destroy (uri);
   }

   bson_free (host_and_port);
   mongoc_cleanup ();
   return res;
}

First launch two separate instances of mongod (must be done from separate shells):

$ mongod

$ mkdir /tmp/db2
$ mongod --dbpath /tmp/db2 --port 27018 # second instance

Now compile and run the example program:

$ cd examples/common_operations/$ gcc -Wall -o example common-operations.c $(pkg-config --cflags --libs libmongoc-1.0)$ ./example localhost:27017 localhost:27018
Inserting data
explain
{
   "executionStats" : {
      "allPlansExecution" : [],
      "executionStages" : {
         "advanced" : 19,
         "direction" : "forward" ,
         "docsExamined" : 76,
         "executionTimeMillisEstimate" : 0,
         "filter" : {
            "x" : {
               "$eq" : 1
            }
         },
         "invalidates" : 0,
         "isEOF" : 1,
         "nReturned" : 19,
         "needTime" : 58,
         "needYield" : 0,
         "restoreState" : 0,
         "saveState" : 0,
         "stage" : "COLLSCAN" ,
         "works" : 78
      },
      "executionSuccess" : true,
      "executionTimeMillis" : 0,
      "nReturned" : 19,
      "totalDocsExamined" : 76,
      "totalKeysExamined" : 0
   },
   "ok" : 1,
   "queryPlanner" : {
      "indexFilterSet" : false,
      "namespace" : "test.things",
      "parsedQuery" : {
         "x" : {
            "$eq" : 1
         }
      },
      "plannerVersion" : 1,
      "rejectedPlans" : [],
      "winningPlan" : {
         "direction" : "forward" ,
         "filter" : {
            "x" : {
               "$eq" : 1
            }
         },
         "stage" : "COLLSCAN"
      }
   },
   "serverInfo" : {
      "gitVersion" : "05552b562c7a0b3143a729aaa0838e558dc49b25" ,
      "host" : "MacBook-Pro-57.local",
      "port" : 27017,
      "version" : "3.2.6"
   }
}