MongoDB sort does not work - mongodb

Question

since this morning (I think) I can't sort my collection. The collection has 26944 objects inside (it is currently my biggest - but that cant be the reason right?).
So what i'm doing:
myCollection.find().sort({ _id: 1 })
this works great and without any problem!
but:
myCollection.find().sort({ xxx: 1 })
each other case don't work, xxx means any other key on my object it does not matter which I take...
for example:
myCollection.find({modified: { $exists : true } })
Returns an result with 26944 Objects,
myCollection.find({modified: { $exists : true } }).sort({"modified":-1})
will return 0 Objects.
Did I something wrong?
Maybe I had crashed my collection? Did somebody know what could do in this case? (excepts dropping the collection)
Example Object:
{
"_id": ObjectId("55071e25760e250d050ed8d5"),
"sysModified": new Date("2015-03-15T21:10:12+0100"),
"created": new Date(1426529829922),
"modified": new Date(1426528563945),
"payments": [
],
"orderItems": [
{
"orderId": 'xxxxx',
"itemId": 'xxxxx',
"ean": "XXXXX"
"quantity": 1,
"name": "xxxxxxx",
"vat": 19,
"price": 29.989999999999998437,
"currency": "EUR",
"_id": ObjectId("5507365c89866d820dcef7e0"),
},
{
"orderId": 'xxxxx',
"itemId": 'xxxxx',
"ean": "XXXXX"
"quantity": 1,
"name": "xxxxxxx",
"vat": 19,
"price": 29.989999999999998437,
"currency": "EUR",
"_id": ObjectId("5507365c89866d820dcef7e1"),
}
],
"orderData": {
"orderId": XXXXXXXXX,
"type": "order",
"status": "XXXXXXXXX",
"timestamp_php": XXXXXXXXX,
"customer": XXXXXXXXX,
...
},
"orderDeliveryAddress": {
"customer": xxxxxxxxxxx,
"company": "",
"additional": "",
"firstName": "xxxxxxxxxxx",
"surname": "xxxxxxxxxxx",
"street": "xxxxxxxxxxx",
"houseNumber": "xxxxxxxxxxx",
"zip": "xxxxxxxxxxx",
"city": "xxxxxxxxxxx",
"iso2": "xxxxxxxxxxx",
"phone": "xxxxxxxxxxx",
"fax": "xxxxxxxxxx",
"email": "xxxxxxxxxxx",
},
"orderCustomerAddress": {
"customer": xxxxxxxxxxx,
"company": "",
"additional": "",
"firstName": "xxxxxxxxxxx",
"surname": "xxxxxxxxxxx",
"street": "xxxxxxxxxxx",
"houseNumber": "xxxxxxxxxxx",
"zip": "xxxxxxxxxxx",
"city": "xxxxxxxxxxx",
"iso2": "xxxxxxxxxxx",
"phone": "xxxxxxxxxxx",
"fax": "xxxxxxxxxx",
"email": "xxxxxxxxxxx",
},
"__v": 1
}

Answer 1

It is possible that your collection is to big. If you try so sort on a key, that has no index this can result in an error if the resultset is to big. The error should look like this:
error: {
"$err" : "too much data for sort() with no index.
add an index or specify a smaller limit",
"code" : 10128
}
What you can do is add an index to your key you want to sort by:
db.myCollection.ensureIndex({"modified":-1})
Or you reduce the resultset before sorting:
myCollection.find({modified: { $exists : true } }).limit(10).sort({"modified":-1})