mongo $slice query reverse index out of range - mongodb

Question

The following query in mongo, behaves strange :
db.items.findOne({},{ "List": { "$slice": [ skip, 3 ] }})
First:
Instead of returning one object with ["_id","List"] keys only, it returns a full object.
Second:
if skip is negative and |skip| is higher than list.length then it returns the first three elements as though skip==0
I would expect for:
{
"_id" : ObjectId("542babf265f5de9a0d5c2928"),
"List" : [
1,
2,
3,
4,
5
]
"other" : "not_important"
}
query:
db.items.findOne({},{ "List": { "$slice": [-10, 3 ] }})
to get:
{
"_id" : ObjectId("542babf265f5de9a0d5c2928"),
"List" : []
}
instead, I get:
{
"_id" : ObjectId("542babf265f5de9a0d5c2928"),
"List" : [
1,
2,
3
]
"other" : "not_important"
}
Why?
I use mongoDB 2.4.10

Answer 1

Second: if skip is negative and |skip| is higher than list.length then it returns the first three elements as though skip==0
Yes. That is how the javascript Array.prototype.slice() method works, which is internally used by mongodb.
According to the ECMAScript® Language Specification,
If relativeStart is negative, let k be max((len + relativeStart),0);
else let k be min(relativeStart, len).
In your case relativeStart is -10,
k = max((-10+5),0), k = 0; (where, 5 is the length of your array).
Hence k or skip will always be 0, in these cases.
First: Instead of returning one object with ["_id","List"] keys only, it returns a full object.
Yes, the projection operator works that way. Unless a inclusion or exclusion is explicitly specified in the projection parameter, the whole document is retrieved with the projection operators such as $slice,$elemmatch being applied.
db.items.findOne({},{"_id":1,"List": { "$slice": [-10, 3 ] }})
would return:
{ "_id" : ObjectId("542babf265f5de9a0d5c2928"), "List" : [ 1, 2, 3 ] }
The second parameter to the findOne() method is not only for simple projection purpose, fields are not projected, only if any one of the field names have a value of 0 or 1 against them. If not the whole document is returned. If any field has a projection operator to be applied, it would be applied and projected.
The projection mechanism seems to happen in the below manner, whenever the $slice operator is involved.
By default all the fields would be included for projection.
By Default all the fields whose values are derived based on the projection operator, $slice, if truthy, are always displayed, irrespective of the below.
Steps taking place for exclusion or inclusion.
The list of fields specified in the projection parameter are accumulated in their specified order.
For only the first field encountered with value '0' or '1':
If the
field has a value '0' - then it is excluded, and all the remaining
fields are marked to be included.
If a field has '1' - then it is included, and all the remaining fields
are marked to be excluded.
For all the subsequent fields, they are excluded or included based on
their values.

Answer 2

Whilst this behavior is by design for the $slice operator, it is possible since MongoDB 3.2 to evaluate this and alter the result with the aggregation operator for $slice:
Given the example documents:
{ "_id" : ObjectId("5922846dbcf60428d0f69f6e"), "a" : [ 1, 2, 3, 4 ] }
{ "_id" : ObjectId("5922847cbcf60428d0f69f6f"), "a" : [ 5, 6 ] }
If given a conditional expression to test against the length of the array with $size and only perform the $slice when the reverse index was greater than or equal to that length, or otherwise return an empty array:
db.collection.aggregate([
{ "$project": {
"a": {
"$cond": {
"if": { "$gte": [ { "$size": "$a" }, 4 ] },
"then": { "$slice": [ "$a", -4, 2 ] },
"else": { "$literal": [] },
}
}
}}
])
Then of course you get:
{ "_id" : ObjectId("5922846dbcf60428d0f69f6e"), "a" : [ 1, 2 ] }
{ "_id" : ObjectId("5922847cbcf60428d0f69f6f"), "a" : [ ] }
So that is how you could get MongoDB to return a "slice" that acts in this way.