Utilities for storing and retrieving files from the database.
GridFS is a storage specification all supported drivers implement. Basically, it defines two collections: files, for file metadata, and chunks, for file content. If the file is large, it will automatically be split into smaller chunks and each chunk will be saved as a document in the chunks collection.
Each document in the files collection contains the filename, upload date, and md5 hash. It also contains a unique _id field, which can be used to query the chunks collection for the file's content. Each document in the chunks collection contains a chunk of binary data, a files_id field that matches its file's _id, and the position of this chunk in the overall file.
For example, the files document is something like:
array("_id" => 123456789, "filename" => "foo.txt", "chunkSize" => 3, "length" => 12);
array("files_id" => 123456789, "n" => 0, "data" => new MongoBinData("abc"));
array("files_id" => 123456789, "n" => 1, "data" => new MongoBinData("def"));
array("files_id" => 123456789, "n" => 2, "data" => new MongoBinData("ghi"));
array("files_id" => 123456789, "n" => 3, "data" => new MongoBinData("jkl"));
You should be able to use any files created by MongoGridFS with any other drivers, and visa vera. However, some drivers expect that all metadata associated with a file be in a "metadata" field. If you're going to be using other languages, it's a good idea to wrap info you might want them to see in a "metadata" field. For example, instead of:
$grid->storeFile("somefile.txt", array("date" => new MongoDate()));
use something like:
$grid->storeFile("somefile.txt", array("metadata" => array("date" => new MongoDate())));
MongoGridFS represents the files and chunks collections. MongoGridFS extends MongoCollection, and an instance of MongoGridFS has access to all of MongoCollection methods, which act on the files collection:
$grid = $db->getGridFS();
$grid->update(array("filename" => "foo"), $newObj); // update on the files collection
Another example of manipulating metadata:
// save a file
$id = $grid->storeFile("game.tgz");
$game = $grid->findOne();
// add a downloads counter
$game->file['downloads'] = 0;
// increment the counter
$grid->update(array("_id" => $id), array('$inc' => array("downloads" => 1)));
You can also access the chunks collection from an instance of MongoGridFS:
$chunks = $grid->chunks; // $chunks is a normal MongoCollection
$chunks->insert(array("x" => 4));
There are some methods for MongoGridFS with the same name as MongoCollection methods, that behave slightly differently. For example, MongoGridFS::remove() will remove any objects that match the criteria from the files collection and their content from the chunks collection.
To store something new in GridFS, there are a couple options. If you have a filename, you can say:
$grid->storeFile($filename, array("whatever" => "metadata", "you" => "want"));
If you have a string of bytes that isn't a file, you can also store that using MongoGridFS::storeBytes():
$grid->storeBytes($bytes, array("whatever" => "metadata", "you" => "want"));
Querying a MongoGridFS collection returns a MongoGridFSCursor, which behaves like a normal MongoCursor except that it returns MongoGridFSFiles instead of associative arrays.
MongoGridFSFiles can be written back to disc using MongoGridFSFile::write() or retrieved in memory using MongoGridFSFile::getBytes(). There is currently no method that automatically streams chunks, but it would be fairly easy to write by querying the $grid->chunks collection.
MongoGridFSFile objects contain a field file which contains any file metadata.
MongoDB core docs on » GridFS. There's also a nice intro to » saving user uploads and » adding metadata at LightCubeSolutions.com.