$filter

db.inventory.aggregate(
[
{
$unwind: "$sizes"
}
,
{
$group:{"}
}
] )

sudo iptables -L -n
service iptables status
service iptables start

$project: {
items: {
$filter: {
input: "$items",
as: "item",
cond: { $gte: [ "$$item.price", 100 ] }
}
}
}

msg: {$filter: {input: "$msg", as: "msg", cond: {$eq: ["$$msg.name", "name1"]}}}
 $filter: {
     input: [ 1, "a", 2, null, 3.1, NumberLong(4), "5" ],
     as: "num",
     cond: { $and: [
        { $gte: [ "$$num", NumberLong("-9223372036854775807") ] },
        { $lte: [ "$$num", NumberLong("9223372036854775807") ] }
      ] }
  }

db.collectionname.aggregate({$match: {"msg.name": "name1"}}, {$project: {_id: 0, timestamp: "$timestamp", msg: {$filter: {input: "$msg", as: "msg", cond: {$eq: ["$$msg.name", "name1"]}}}}})

$filter

原文：http://www.cnblogs.com/jayruan/p/5867711.html