Adding array title to Linq-to-SQL-to-JSON in Web API

I am using Linq to SQL in a Web API web service to retrieve data from a database and return a JSON file.

My question is actually pretty simple, but I have been through the forums and couldn't find an answer. Please find below the description of my issue, as well as my (simplified) sourcecode.

The objects I return have two levels of data. To make you understand, here is how my classes look like :

public class X
{
    public int ID { get; set; }
    public DateTime date { get; set; }

    public virtual ICollection<Y> Ys
    public virtual ApplicationUser User { get; set; }
}

public class Y
{
    public int ID { get; set; }
    public int XID { get; set; }
    public decimal Price { get; set; }
    public int Quantity { get; set; }

    public virtual X x { get; set; }
}

You can see that for each X1 object, I can have several X2 objects nested.

To retrieve that, I use the following Linq to SQL in my WebAPI Controller :

public IHttpActionResult GetXsByUser(string userID)
    {
        var xs = (from x in db.Xs
                        where x.User.Id == userID
                        orderby x.date
                        select new
                        {
                            x_id = x.ID,
                            date = x.date,
                            Ys = (from y in db.Ys
                                         where x.User.Id == userID && x1.ID == y.XID
                                         select new
                                         {
                                             unit_price = y.Price,
                                             quantity = y.Quantity
                                         })
                        });
        if (xs == null)
        {
            return NotFound();
        }
        return Ok(xs);
}

My web service works fine and returns the following JSON :

[
  {
    "$id": "1",
    "x_id": 1,
    "date": "2014-01-24T00:00:00",
    "Ys": [
      {
        "$id": "2",
        "unit_price": 2.47,
        "quantity": 2
      },
      {
        "$id": "3",
        "unit_price": 1.25,
        "quantity": 3
      },
      {
        "$id": "4",
        "unit_price": 1.99,
        "quantity": 2
      }
    ]
  },
  {
    "$id": "5",
    "x_id": 2,
    "date": "2014-01-28T00:00:00",
    "Ys": [
      {
        "$id": "6",
        "unit_price": 6.22,
        "quantity": 1
      },
      {
        "$id": "7",
        "unit_price": 1.2,
        "quantity": 3
      }
    ]
  }
]

The problem is, to then deserialize this in my mobile app, I have to use classes as follows :

public class Y
{
    public string _$id { get; set; }
    public double unit_price { get; set; }
    public int quantity { get; set; }
}

public class RootObject
{
    public string _$id { get; set; }
    public int x_id { get; set; }
    public string date { get; set; }
    public List<Y> Ys { get; set; }
}

But i would like to be able to use classes as follow :

public class Y
{
    public string _$id { get; set; }
    public double unit_price { get; set; }
    public int quantity { get; set; }
}

public class OnlineX
{
    public string _$id { get; set; }
    public int x_id { get; set; }
    public string date { get; set; }
    public List<Y> Ys { get; set; }
}

public class RootObject
{
    public List<OnlineX> OnlineXs { get; set; }
}

I have worked with a JSON editor and know that the solution to get this is to have the following JSON file instead of the previous one :

{
  "OnlineXs": [
    {
      "$id": "1",
      "x_id": 1,
      "date": "2014-01-24T00:00:00",
      "Ys": [
        {
          "$id": "2",
          "unit_price": 2.47,
          "quantity": 2
        },
        {
          "$id": "3",
          "unit_price": 1.25,
          "quantity": 3
        },
        {
          "$id": "4",
          "unit_price": 1.99,
          "quantity": 2
        }
      ]
    },
    {
      "$id": "5",
      "x_id": 2,
      "date": "2014-01-28T00:00:00",
      "Ys": [
        {
          "$id": "6",
          "unit_price": 6.22,
          "quantity": 1
        },
        {
          "$id": "7",
          "unit_price": 1.2,
          "quantity": 3
        }
      ]
    }
  ]
}

Notice that the only thing that changes is that I add a title to my array of Xs ("Online Xs"). That is why I said that my question is simple. But the thing is, I have no idea how to do that in Web API. Is it just a small change in my Linq to SQL request? Should i build a custom JSON serializer?

I hope that my question is clear enough, and if you want some more information, I'll be happy to provide them.

Thanks a lot in advance

EDIT :

Ok, I've found the solution, it was simple indeed. Here it is :

I had to replace :

return Ok(xs);

by

return Ok(new { OnlineXs = xs });