How can I query MongoDB for an embedded document via the C# driver?

Question

I'm wondering how one can query a MongoDB collection for an embedded document (in an array) via the official C# driver version 1.7? By querying for an embedded document I mean I'd like to retrieve only the embedded document and not the one containing it, AKA projection.

Example of the kind of data model, with embedded documents, I am querying:

// Library, contained in db.Libraries
{
    _id: 1,
    Categories: [{_id: 2, Name: "Classics", Books: [{_id: 3, Name: The Count of Monte Cristo}]}]
}

The problem here would be how to query the Library collection for an object with _id 1 and one of its books with _id 3, and return only the Book. Is it doable at all? As far as I could tell, it would be possible to do this through a projection in the MongoDB shell.

My example query would be to query db.Libraries for a Book with _id 3 contained in a Library with _id 1. The Book returned would be this sub-document:

{_id: 3, Name: The Count of Monte Cristo}]}

I've looked at the question How to retrieve an embedded document using the official C# driver for MongoDB?, but I can't make the accepted answer work for me.

EDIT:

I can see now that the accepted answer of How to retrieve an embedded document using the official C# driver for MongoDB? works, kind of. I failed to see that it iterates over each found document, equivalent to this:

var libraryCollection = new MongoCollection<Library>();
var refBook = new Book { Id = ObjectId.GenerateNewId().ToString(), Name = "The Count of Monte Cristo" };
libraryCollection.Insert(new Library { Id = ObjectId.GenerateNewId().ToString(), Categories = new[] { new Category { Books = new[] { refBook } } } });

MongoCursor<Library> libraries = libraryCollection.Find(new QueryDocument(
                    new Dictionary<string, object>
                        {
                            {"_id", new ObjectId()},
                        }
                    ));
Book book;
foreach (var library in libraries)
{
    book = library.Categories.SelectMany(c => c.Books).FirstOrDefault(b => b.Id == refBook.Id);
}

However, this solution is moot since it retrieves whole Library documents rather than just the embedded Book document. I'm really after having to deserialize only the embedded Book, AKA projection.

How can you expect someone to help you when the "Accepted Answer" in the link that you provided truly shows a lot more code than you have provided.. please show all relevant code that pertains to your question otherwise you are asking us to be amazing Mind Readers — MethodMan
– MethodMan, Commented Jan 26, 2013 at 15:26
@DJKRAZE No need to be rude. The problem itself should be clear from the data model I have provided and what I am querying for (which I have also stated). Can you specify what kind of code is missing to explain the problem? — aknuds1
– aknuds1, Commented Jan 26, 2013 at 15:44
I am not being rude I am asking you to show the code that you have where you are trying to do the query.. you are only showing what appears to be JSON Script.. — MethodMan
– MethodMan, Commented Jan 26, 2013 at 15:48
@DJKRAZE The "JSON Script" as you call it is the MongoDB's representation of the data model. If I knew how to code the desired query in C# I wouldn't have to ask... Data projections in MongoDB are not trivial, AFAICT they were not possible in C# at some point, but I am hoping they are now. — aknuds1
– aknuds1, Commented Jan 26, 2013 at 15:51
are you saying that something like this won't work..? var query = Query.And(Query.EQ("Books._id", new ObjectId("1"))); MongoCollection<Book> collection = _database.GetCollection<Question>("Books"); MongoCursor<Book> cursor = collection.Find(query); — MethodMan
– MethodMan, Commented Jan 26, 2013 at 16:03

JohnnyHK · Accepted Answer · 2013-01-26 22:23:52Z

To perform a projection where the resulting document is not just filtered, but reshaped, you need to use Aggregate instead of one of the Find methods like this:

var result = collection.Aggregate(
    // Only include the document where _id = 1
    new BsonDocument {{"$match", new BsonDocument {{"_id", 1}}}},
    // 'Unwind' the Categories array by duplicating the docs, one per element.
    new BsonDocument {{"$unwind", "$Categories"}},
    // Now do the same for the Books array inside each Categories element.
    new BsonDocument {{"$unwind", "$Categories.Books"}},
    // Only include the resulting docs with a Book _id of 3
    new BsonDocument {{"$match", new BsonDocument {{"Categories.Books._id", 3}}}},
    // Reshape the document to bring the book attributes out to the top level 
    new BsonDocument {{"$project", new BsonDocument {
        {"_id", "$Categories.Books._id"},
        {"Name", "$Categories.Books.Name"}
    }}}
);

result.Response.toJson():

{"result": [{ "_id": 3.0, "Name": "The Count of Monte Cristo" }], "ok": 1.0 }

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