MongoDb - Find a specific obj within nested arrays - arrays

Question

I hope someone can shed some light on this issue that I have, it's driving me crazy to a point that I have been spending the past three days, learning more and more about mongoDB but still can't figure out this simple query.
What I need to do is to get the object containing the "carId" = "3C".
In other words the object that I want the query to return is:
{
"carId" : "3C",
"_id" : ObjectId("51273329b64f07a40ef1c15e")
}
Here is the dataset (cars):
{
"_id" : ObjectId("56223329b64f07a40ef1c15c"),
"username" : "john",
"email" : "john#john.com",
"accounts" : [
{
"_id" : ObjectId("56322329b61f07a40ef1c15d"),
"cars" : [
{
"carId" : "6A",
"_id" : ObjectId("56323329b64f07a40ef1c15e")
},
{
"carId" : "6B",
"_id" : ObjectId("56323329b64f07a40ef1c15e")
}
]
}
]
},
{
"_id" : ObjectId("56223125b64f07a40ef1c15c"),
"username" : "paul",
"email" : "paul#paul.com",
"accounts" : [
{
"_id" : ObjectId("5154729b61f07a40ef1c15d"),
"cars" : [
{
"carId" : "5B",
"_id" : ObjectId("56323329854f07a40ef1c15e")
}
]
},
{
"_id" : ObjectId("56322117b61f07a40ef1c15d"),
"cars" : [
{
"carId" : "6G",
"_id" : ObjectId("51212929b64f07a40ef1c15e")
},
{
"carId" : "3C",
"_id" : ObjectId("51273329b64f07a40ef1c15e")
},
{
"carId" : "4N",
"_id" : ObjectId("51241279b64f07a40ef1c15e")
}
]
}
]
}
Please note that I have two nested arrays, and apparently MongoDb lacks when it comes to dealing with Projections with deep arrays. The $ operator can only be used once in a projection; leaving with no clues as how to to achieve this simple task.
So again I want to find --only-- the document that has "carId" : "3C" and only return the immediate obj containing the "carId" : "3C". but not the parent objects.
Any help would be so much appreciated. Possibly using either direct MongoDb or Mongoose. Mongoose would be preferred.
As for reference, I have already covered these other related issues wasn't able to figure it out.
Updating a deep record in MongoDb
How to Update Multiple Array Elements in mongodb
Hope in the future, this question and your solutions will help others.

Answer 1

Amir,
You must use the Aggregation Framework. You can build a pipeline that processes a stream documents through several building blocks: filtering, projecting,grouping,sorting,etc.
When dealing with nested arrays you will have to use the $unwind command. You can get what you want by doing the following.
db.cars.aggregate(
//De-normalized the nested array of accounts
{"$unwind": "$accounts"},
//De-normalized the nested array of cars
{"$unwind": "$accounts.cars"},
//match carId to 3C
{"$match": {"accounts.cars.carId" : "3C"}},
//Project the accoutns.cars object only
{"$project" : {"accounts.cars" : 1}},
//Group and return only the car object
{"$group":{"_id":"$accounts.cars"}}
).pretty();
You can use the aggregation framework for "array filtering" by using $unwind .
You can delete each line from the bottom of each command in the aggregation pipeline in the above code to observe the pipelines behavior.

Answer 2

Here's an example without the aggregation framework. I don't think there's a way purely from querying that you'll be able to get just the individual nested object you're looking for so you have to do a little post processing work. something like Mongoose may provide a way to do this but I'm not really up on what the Mongoose API's look like currently.
var doc = db.cars.findOne({"accounts.cars" : {$elemMatch: {"carId" : "3C"}}}, {"accounts.cars.$": 1, _id: 0})
var car = doc.accounts[0].cars[0]