Finding a Document

The Basics

The following code example creates a new mongoc_client_t that we will use to communicate with MongoDB. The Connection String URI component is quite comprehensive.

Using our mongoc_client_t, we get a handle to a mongoc_collection_t which represents the remote collection. query is initialized to a new BSON document that contains no elements, therefore matching every document in the MongoDB collection.

Using the returned cursor, we convert each resulting document to a MongoDB Extended JSON string and print it to stdout.

Lastly, we release all of our heap allocated structures.

<file>example1.c</file> Print all documents in a collection.


#include 
#include 

int
main (int   argc,
      char *argv[])
{
    mongoc_client_t *client;
    mongoc_collection_t *collection;
    mongoc_cursor_t *cursor;
    const bson_t *doc;
    bson_t *query;
    char *str;

    mongoc_init ();

    client = mongoc_client_new ("mongodb://localhost:27017/");
    collection = mongoc_client_get_collection (client, "test", "test");
    query = bson_new ();
    cursor = mongoc_collection_find (collection, MONGOC_QUERY_NONE, 0, 0, 0, query, NULL, NULL);

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

    bson_destroy (query);
    mongoc_cursor_destroy (cursor);
    mongoc_collection_destroy (collection);
    mongoc_client_destroy (client);

    return 0;
}
]]>

Let's use GCC and pkg-config to compile example1.c.

gcc -o example1 example1.c $(pkg-config --cflags --libs libmongoc-1.0)

When using the MongoDB C Driver, you must call mongoc_init() at the beginning of your application. This allows the driver to initialize it's required subsystems. Failure to do so will result in a runtime crash.

Now let's run it!

./example1

Oops, nothing to print! Let's insert a document into our collection using the MongoDB shell.

[christian@starpulse ~]$ mongo MongoDB shell version: 2.4.10 connecting to: test > use test switched to db test > db.test.insert({'hello': 'world'}) > bye

And now let's run it again!

./example1 { "_id" : { "$oid" : "534cde1a4f05ea4055d4cd4c" }, "hello" : "world" }

Congratulations on your first query with the MongoDB C Driver!

Specifying a Query

The above example is useful, but wouldn't it be more useful if we could find a specific document? Let's modify the above program to find a specific document matching {"hello": "world"}.

<file>example2.c</file> Finding a specific document.


#include 
#include 

int
main (int   argc,
      char *argv[])
{
    mongoc_client_t *client;
    mongoc_collection_t *collection;
    mongoc_cursor_t *cursor;
    const bson_t *doc;
    bson_t *query;
    char *str;

    mongoc_init ();

    client = mongoc_client_new ("mongodb://localhost:27017/");
    collection = mongoc_client_get_collection (client, "test", "test");
    query = bson_new ();
    BSON_APPEND_UTF8 (query, "hello", "world");

    cursor = mongoc_collection_find (collection, MONGOC_QUERY_NONE, 0, 0, 0, query, NULL, NULL);

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

    bson_destroy (query);
    mongoc_cursor_destroy (cursor);
    mongoc_collection_destroy (collection);
    mongoc_client_destroy (client);

    return 0;
}
]]>

gcc -o example2 example2.c $(pkg-config --cflags --libs libmongoc-1.0) ./example2 { "_id" : { "$oid" : "534cde1a4f05ea4055d4cd4c" }, "hello" : "world" }

Our document still matches the query, so it is still printed to stdout. Try changing the element values to see how the results change.