Query for field in subdocument - mongodb

Question

An example of the schema i have;
{ "_id" : 1234,
“dealershipName”: “Eric’s Mongo Cars”,
“cars”: [
{“year”: 2013,
“make”: “10gen”,
“model”: “MongoCar”,
“vin”: 3928056,
“mechanicNotes”: “Runs great!”},
{“year”: 1985,
“make”: “DeLorean”,
“model”: “DMC-12”,
“vin”: 8056309,
“mechanicNotes”: “Great Scott!”}
]
}
I wish to query and return only the value "vin" in "_id : 1234". Any suggestion is much appreciated.

Answer 1

You can use the field selection parameter with dot notation to constrain the output to just the desired field:
db.test.find({_id: 1234}, {_id: 0, 'cars.vin': 1})
Output:
{
"cars" : [
{
"vin" : 3928056
},
{
"vin" : 8056309
}
]
}
Or if you just want an array of vin values you can use aggregate:
db.test.aggregate([
// Find the matching doc
{$match: {_id: 1234}},
// Duplicate it, once per cars element
{$unwind: '$cars'},
// Group it back together, adding each cars.vin value as an element of a vin array
{$group: {_id: '$_id', vin: {$push: '$cars.vin'}}},
// Only include the vin field in the output
{$project: {_id: 0, vin: 1}}
])
Output:
{
"result" : [
{
"vin" : [
3928056,
8056309
]
}
],
"ok" : 1
}

Answer 2

If by query, you mean get the values of the vins in javascript, you could read the json into a string called theString (or any other name) and do something like:
var f = [], obj = JSON.parse(theString);
obj.cars.forEach(function(item) { f.push(item.vin) });
If your json above is part of a larger collection, then you'd need an outer loop.