select only subdocuments or arrays - mongodb

Question

{
"_id":{
"oid":"4f33bf69873dbc73a7d21dc3"
},
"country":"IND",
"states":[{
"name":"orissa",
"direction":"east",
"population":41947358,
"districts":[{
"name":"puri",
"headquarter":"puri",
"population":1498604
},
{
"name":"khordha",
"headquarter":"bhubaneswar",
"population":1874405
}
]
},
{
"name":"andhra pradesh",
"direction":"south",
"population":84665533,
"districts":[{
"name":"rangareddi",
"headquarter":"hyderabad",
"population":3506670
},
{
"name":"vishakhapatnam",
"headquarter":"vishakhapatnam",
"population":3789823
}
]
}
]
}
In above collection(i.e countries) i have only one document , and i want to fetch the details about a particular state (lets say "country.states.name" : "orissa" ) ,But i want my result as here under instead of entire document .Is there a way in Mogo...
{
"name": "orissa",
"direction": "east",
"population": 41947358,
"districts": [
{
"name": "puri",
"headquarter": "puri",
"population": 1498604
},
{
"name": "khordha",
"headquarter": "bhubaneswar",
"population": 1874405
}
]
}
Thanks

Answer 1

Tried this:
db.countries.aggregate(
{
"$project": {
"state": "$states",
"_id": 0
}
},
{
"$unwind": "$state"
},
{
"$group": {
"_id": "$state.name",
"state": {
"$first": "$state"
}
}
},
{
"$match": {
"_id": "orissa"
}
}
);
And got:
{
"result" : [
{
"_id" : "orissa",
"state" : {
"name" : "orissa",
"direction" : "east",
"population" : 41947358,
"districts" : [
{
"name" : "puri",
"headquarter" : "puri",
"population" : 1498604
},
{
"name" : "khordha",
"headquarter" : "bhubaneswar",
"population" : 1874405
}
]
}
}
],
"ok" : 1

Answer 2

You can't do it right now, but you will be able to with $unwind in the aggregation framework. You can try it now with the experimental 2.1 branch, the stable version will come out in 2.2, probably in a few months.

Answer 3

Any query in mongodb always return root document.
There is only one way for you to load one sub document with parent via $slice if you know ordinal number of state in nested array:
// skip ordinalNumberOfState -1, limit 1
db.countries.find({_id: 1}, {states:{$slice: [ordinalNumber -1 , 1]}})
$slice work in default order (as documents was inserted in nested array).
Also if you don't need fields from a country you can include only _id and states in result:
db.countries.find({_id: 1}, {states:{$slice: [ordinalNumber -1 , 1]}, _id: 1})
Then result document will looks like this one:
{
"_id":{
"oid":"4f33bf69873dbc73a7d21dc3"
},
"states":[{
"name":"orissa",
"direction":"east",
"population":41947358,
"districts":[{
"name":"puri",
"headquarter":"puri",
"population":1498604
},
{
"name":"khordha",
"headquarter":"bhubaneswar",
"population":1874405
}
]
}]
}

Answer 4

db.countries.find({ "states": { "$elemMatch": { "name": orissa }}},{"country" : 1, "states.$": 1 })

Answer 5

If you don't want to use aggregate, you can do it pretty easily at the application layer using underscore (included by default):
var country = Groops.findOne({"property":value);
var state _.where(country, {"state":statename});
This will give you the entire state record that matches statename. Very convenient.