How to filter array in subdocument with MongoDB

I have array in subdocument like this

{
	"_id" : ObjectId("512e28984815cbfcb21646a7"),
	"list" : [
		{
			"a" : 1
		},
		{
			"a" : 2
		},
		{
			"a" : 3
		},
		{
			"a" : 4
		},
		{
			"a" : 5
		}
	]
}

Can I filter subdocument for a > 3

My expect result below

{
	"_id" : ObjectId("512e28984815cbfcb21646a7"),
	"list" : [
		{
			"a" : 4
		},
		{
			"a" : 5
		}
	]
}

I try to use $elemMatch but returns the first matching element in the array

My query:

db.test.find( { _id" : ObjectId("512e28984815cbfcb21646a7") }, { 
	list: { 
		$elemMatch: 
			{ a: { $gt:3 } 
			} 
	} 
} )

The result return one element in array

{ "_id" : ObjectId("512e28984815cbfcb21646a7"), "list" : [ { "a" : 4 } ] }

and I try to use aggregate with $match but not work

db.test.aggregate({$match:{_id:ObjectId("512e28984815cbfcb21646a7"), 'list.a':{$gte:5}  }})

It's return all element in array

{
    "_id" : ObjectId("512e28984815cbfcb21646a7"),
    "list" : [
        {
            "a" : 1
        },
        {
            "a" : 2
        },
        {
            "a" : 3
        },
        {
            "a" : 4
        },
        {
            "a" : 5
        }
    ]
}

Can I filter element in array to get result as expect result?

Using aggregate is the right approach, but you need to $unwind the list array before applying the $match so that you can filter individual elements and then use $group to put it back together:

db.test.aggregate([
    { $match: {_id: ObjectId("512e28984815cbfcb21646a7")}},
    { $unwind: '$list'},
    { $match: {'list.a': {$gt: 3}}},
    { $group: {_id: '$_id', list: {$push: '$list.a'}}}
])

outputs:

{
  "result": [
    {
      "_id": ObjectId("512e28984815cbfcb21646a7"),
      "list": [
        4,
        5
      ]
    }
  ],
  "ok": 1
}

MongoDB 3.2 Update

Starting with the 3.2 release, you can use the new $filter aggregation operator to do this more efficiently by only including the list elements you want during a $project:

db.test.aggregate([
    { $match: {_id: ObjectId("512e28984815cbfcb21646a7")}},
    { $project: {
        list: {$filter: {
            input: '$list',
            as: 'item',
            cond: {$gt: ['$$item.a', 3]}
        }}
    }}
])

Above solution works best if multiple matching sub documents are required. $elemMatch also comes in very use if single matching sub document is required as output

db.test.find({list: {$elemMatch: {a: 1}}}, {'list.$': 1})

Result:

{
  "_id": ObjectId("..."),
  "list": [{a: 1}]
}

Use $filter aggregation

> Selects a subset of the array to return based on the specified > condition. Returns an array with only those elements that match the > condition. The returned elements are in the original order.

db.test.aggregate([
    {$match: {"list.a": {$gt:3}}}, // <-- match only the document which have a matching element
    {$project: {
        list: {$filter: {
            input: "$list",
            as: "list",
            cond: {$gt: ["$$list.a", 3]} //<-- filter sub-array based on condition
        }}
    }}
]);