Rails 4 Passing Json To Ember

My Json output is not coming the way as expected by Ember. I am getting the following output:

{
   "user":{
       "id":3,
       "email":"user5@user.com"
       "username":"user5"
    },
   "uploads":[{
       "id":5,
       "user_id":3,
       "image_file_name":"image-2.jpg"
    },
    {
        "id":6,
        "user_id":3,
        "image_file_name":"image-4.jpg"
    }]
}    

What I am missing is in the user section of the json output is the upload_ids: [upload ids separated with commas] like this:

"user":{
       "id":3,
       "email":"user5@user.com"
       "username":"user5",
       "upload_ids":[5,6]
},

I have the following in the UserSerializer file:

attributes :id, :email, :username
has_many :uploads, embed: :ids, key: "uploads", include: true

In the users_controller.rb I have the following:

@uploads = @user.uploads.all
respond_with(:user=>@user, :uploads=>@uploads)

I have active model serializer gem version 0.9.2.

Please if someone can help with this, it will be hugely appreciated.