How to retrieve a sub document array in MongoDB - mongodb

Question

I have a collection in mongodb like this:
db.country_list.find().pretty()
{
"_id" : ObjectId("53917321ccbc96175d7a808b"),
"countries" : [
{
"countryName" : "Afghanistan",
"iso3" : "AFG",
"callingCode" : "93"
},
{
"countryName" : "Aland Islands",
"iso3" : "ALA",
"callingCode" : "358"
},
{
"countryName" : "Albania",
"iso3" : "ALB",
"callingCode" : "355"
}
]
}
like that i have 100 country details
i want to retrieve a country name where the calling code is 355.
I have tried like this
db.country_list.find({countries: {$elemMatch :{ 'callingCode':'355'} } } )
and like this
db.country_list.find({'countries.callingCode':'355'}).pretty()
but i am getting all records.How to get a specific record .Thanks in advance

Answer 1

What you want is the positional $ operator:
db.country_list.find(
{ "countries": { "$elemMatch" :{ "callingCode":"355"} } }
{ "countries.$": 1 }
)
Or even with the other syntax you tried:
db.country_list.find(
{ "countries.callingCode": "355"}
{ "countries.$": 1 }
)
This is because a "query" matches documents and is not a filter for the array contained in those documents. So the second argument there projects the field with the "position" that was matched on the query side.
If you need to match more than one array element, then you use the aggregation framework which has more flexibility:
db.country_list.aggregate([
// Matches the documents that "contain" the match
{ "$match": {
"countries.callingCode": "355"
}},
// Unwind the array to de-normalize as documents
{ "$unwind": "$countries" },
// Match to "filter" the array content
{ "$match": {
"countries.callingCode": "355"
}},
// Group back if you want an array
{ "$group": {
"_id": "$_id",
"countries": { "$push": "$countries" }
}}
])
Or with MongoDB 2.6 or greater you can do this without the $unwind and $group:
db.country_list.aggregate([
// Matches the documents that "contain" the match
{ "$match": {
"countries.callingCode": "355"
}},
// Project with "$map" to filter
{ "$project": {
"countries": {
"$setDifference": [
{ "$map": {
"input": "$countries",
"as": "el",
"in": {
"$cond": [
{ "$eq": [ "$$el.callingCode", "355" ] }
"$$el",
false
]
}
}},
[false]
]
}
}}
])