MongoDB: Missing fields after sort() when using projection - mongodb

Question

So I have a database filled with image information, and I want to retrieve a subset of the fields sorted by ascending date. I use the following query to retrieve the aggregated set:
db.images.find({}, {rel_path: 1, date: 1}).sort({'date.year': 1, 'date.month': 1})
I expect this query to return a set looking something like this:
{
"_id": ObjectId("530deb1060832c64291a11a7"),
"date": { "year: 2006, "month": 2 },
"rel_path": "/mnt/backup/Backup/Photos/asdfasdfasdf.jpg"
}
{
"_id": ObjectId("530de1db60832c64291a05ec"),
"date": { "year: 2006, "month": 5 },
"rel_path": "/mnt/backup/Backup/Photos/qweqweqwe.jpg"
}
... <more documents> ...
What I get, however, looks like this:
{
"_id": ObjectId("530deb1060832c64291a11a7"),
"rel_path": "/mnt/backup/Backup/Photos/asdfasdfasdf.jpg"
}
{
"_id": ObjectId("530de1db60832c64291a05ec"),
"rel_path": "/mnt/backup/Backup/Photos/qweqweqwe.jpg"
}
... <more documents> ...
If I skip the 'sort()' I get all fields from my projection, so it seems the 'date' field somehow is removed by the 'sort()' call.
Anyone have any idea what's going on here?
Edit: Here's a sample document by request:
{
"_id" : ObjectId("530de16860832c64291a0562"),
"orientation" : 1,
"camera_make" : "Apple",
"camera_model" : "iPhone 4",
"rel_path" : "Bröllopsbilder/IMG_0997.JPG",
"file_size" : 1827977,
"date" : { "month" : "10", "year" : "2011" },
"root" : "/mnt/backup/Backup/Bilder/",
"md5" : "fb26ebf24914d515144be5e53797744b"
}

Answer 1

The find() query looks fine and it works as expected. I tested it by running it against a similar data set.
Reason this could be happening is when a few documents in the collection do not have the "date" field. Try running the same query by adding a filter criteria in the find query to return only those results where "date" field exists using $exists operator i.e.,:
db.images.find({date:{$exists:true}}, {rel_path: 1, date: 1})
.sort({'date.year': 1, 'date.month': 1})