Re: How to filter array in subdocument with MongoDB

Rodrigo Machado

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Feb 27, 2013, 11:46:10 AM2/27/13

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Hi salanyot,

You can do something like this but I think that this is not very efficient: (Probably another schema design would be better)

db.test.aggregate(

{$match: {_id: ObjectId("512e28984815cbfcb21646a7")}},

{$unwind: '$list'},

{$match: {"list.a": {$gt: 3}}},

{$group: {_id: "$_id", "list": {$push:"$list"}}}

);

The $unwind will separate the elements in the array into its own documents, the we match for what we want, then use a $group with $push to, err, group the elements again into an array.

Hope this helps.

Cheers,

Rodrigo Machado

On Wednesday, February 27, 2013 1:21:55 PM UTC-3, salanyot suwannachoti wrote:

Hello everyone.
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?

Sam

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Aug 1, 2013, 11:16:24 PM8/1/13

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Hi,

I am also having the same problem.

I have Big documents. I need to query on array of elements.
I am getting all elements instead of matched elements in the array.

Do we have any better solution?

Please help me.

Thanks,
Samanth

Asya Kamsky

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Aug 5, 2013, 1:04:09 PM8/5/13

to mongodb-user

You can take a look at $elemMatch projection operator
http://docs.mongodb.org/manual/reference/projection/elemMatch/

That will allow you to return the first matching array element. If you
need multiple elements then it's likely that a different schema might
serve your use case better - in general, embedding is best when you
normally want all or none of the array elements - though it also works
well when you want just one.

Asya

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