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[Retrieve only the queried element in an object array in MongoDB collection](/questions/3985214/retrieve-only-the-queried-element-in-an-object- array-in-mongodb-collection) (20 answers)
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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?
Answer
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: ['$$item.a', 3]}
}}
}}
])
$and: get data between 0-5:
cond: {
$and: [
{ $gt: [ "$$item.a", 0 ] },
{ $lt: [ "$$item.a", 5 ] }
]}