无法使用php更新Mongodb gridfs

I tried to update a document in Mongodb gridfs with the document's ID. Initially my document was :

{
    [_id] => MongoId Object (
            [$id] => 5218a723db8af6920a3c6624
    )
    [Username]   => 'Old Name'
    [Phone]      => 'xxxxxxxxxx'
    [Address]    => 'User address'
    [Email]      => 'User email'
    [DecMak]     => 'Some text'
    [Descr]      => 'my description'
    [ImgName]    => 'Image Name'
    [Str]        => 6
    [Date]       => '25-08-2013'
    [filename]   => 'MyJpg.JPG'
    [uploadDate] => MongoDate Object (
        [sec]  => 1377305640
        [usec] => 262000
    )
    [length]     => 1099792
    [chunkSize]  => 262144
    [md5]        => 2e3bc10f7deeb0334ea0af4bd0fe9bdf
}

And I wanted to update the value of only the 'Username' field. I used the following code to update it :

$m = new MongoClient();
$db = $m->mydb;
$gridfs = $db->getGridFS();
$collection = $db->fs->files;
$cursor = $collection->find(array('_id'=>new MongoID($_POST['_id'])));
     foreach ($cursor as $obj)
     {
     $db->fs->files->update(array('_id'=>new MongoID($_POST['_id'])), 
     //identified the document with the ID.
     array('Username' => $_POST['name']));
     //Original name be changed to what filled in the webform.
     }

But after update What it made to my document is this :

{
    [_id]      => MongoId Object (
         [$id] => 5218a723db8af6920a3c6624
    ),
    [Username] => 'New Name',
}

So all other keys and their values disappeared - What went wrong in my code? Do I need to mention all keys and values pairs while updating a document? I hope it should not be..

Further, what can be the code to update the binary file like image / mp3 etc.

Thanks in advance for your support!

Vasudev

I assume you've moved on from this, but here's the answer in case anyone else is after it.

This allows you to update specific metadata to do with a file, while leaving all the other values alone.

$m = new MongoClient();
$db = $m->mydb;
$gridfs = $db->getGridFS();
$cursor = $gridfs->find(array('_id'=>new MongoID($_POST['_id'])));
foreach ($cursor as $obj) {
    $obj->file['Username'] = $_POST['name'];
    $gridfs->save($obj->file);
}

Since we have the _id here, we could also use $gridfs->findOne(array('_id' => new MongoID($_POST['_id']))); to return the $obj directly instead of a cursor. If you do this, just be sure to test that $obj isn't null (no document found) before you try to update the file.

$m = new MongoClient();
$db = $m->mydb;
$gridfs = $db->getGridFS();
$obj = $gridfs->findOne(array('_id'=>new MongoID($_POST['_id'])));
if ($obj) {
    $obj->file['Username'] = $_POST['name'];
    $gridfs->save($obj->file);
}