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Operating System:
ALL
Steps To Reproduce:

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db.fs.update(
{x:4}
,{ $push : { y : { $each : [
{a:5}
,
{a:1}
,
{a:4}
,
{a:2}
,
{a:3}
], $slice:-4, $sort:
{a:1}
, $sort:
{a:-1}
} } })

db.fs.find()
{ "_id" : ObjectId("50fd0989a8f4e3eef998f845"), "x" : 4, "y" : [
{ "a" : 3 }
,
{ "a" : 2 }
,
{ "a" : 2 }
,
{ "a" : 1 }
] }

Show
db.fs.update( {x:4} ,{ $push : { y : { $each : [ {a:5} , {a:1} , {a:4} , {a:2} , {a:3} ], $slice:-4, $sort: {a:1} , $sort: {a:-1} } } }) db.fs.find() { "_id" : ObjectId("50fd0989a8f4e3eef998f845"), "x" : 4, "y" : [ { "a" : 3 } , { "a" : 2 } , { "a" : 2 } , { "a" : 1 } ] }
Confidence Status:
None
Work Order:
3

Using $push with $slice and $sort causes the wrong elements to be stored in the fixed size array.