Ember PUT request using JSON is giving a weird error - ruby-on-rails

Question

(If the next is too much to read for you, please read the last line). I'm using Ember and Rails backend from a while, but I'm still a bit stick with transactions or what other way is used to send actions to the backend. I have Users and Events. My Users can participate in one or several Events, and for each Event several Users can attend. I'm maintaining this relationship using the Mongoid macro has_and_belongs_to_many in this way:
Event
# ...
has_and_belongs_to_many :attendings, class_name: "User" , inverse_of: :attendances
# ...
User
# ...
has_and_belongs_to_many :attendances, class_name: "Event" , inverse_of: :attendings
# ...
In my template I'm showing a list of all the Events stored in my backend and a button "Attend" if the User is not already attending, and "Not attend" it the User is attending. For attending, I've tried to make the PUT request using transactions, making the next steps in my Ember Event controller:
Event controller (Ember), actionAttending method using transactions:
actionAttending: function() {
var userId = this.get('controllers.app.model.id');
this.transaction = this.get('store').transaction();
this.get('attendings').pushObject(App.User.find(userId));
this.transaction.add(this.get('model'));
this.transaction.commit();
}
When I try this, my Event JSON request is containing all the parameters and relations but attendings attribute. So I decided to try using a jQuery request:
Event controller (Ember), actionAttending method using jQuery ajax:
// ...
var eId = this.get('id');
this.get('attendings').pushObject(App.User.find(userId));
var eAttendings = this.get('attendings');
var url = "/events/" + eId + ".json";
var data = { event: { id: eId, attendings: eAttendings } };
$.ajax({
type: "PUT",
url: url,
data: data,
dataType: "JSON"
});
// ...
Well, this is pretty much working, except for the line where I declare eAttendings, where Ember complains about init function is not called, or somehing similar. After googling a bit I found a "solution", which is convert to array, so the line changed like this:
var eAttendings = this.get('attendings').toArray();
My nice error now is:
TypeError: fullName is undefined
var nameParts = fullName.split(":"),
This is raised in Firebug before my ajax request is launched. I don't know what's the exact meaning or how to fix it...
Anyway, I wanted to try if my ajax request is working, so I tried it in curl:
curl --request PUT localhost:3000/events/521b97ef5ef9095ba211bf70 --data "id=521b97ef5ef9095ba211bf70&attendings=521b7eda99027121d1533015"
The answer is:
{"errors":{"attendings":["is invalid"]}}
And the bakend is returning a 422 Unproccesable entity answer... I've got no validations on my model on this field, so I don't know what's going on here either... My update action on Rails Event controller is like this:
def update
e = Event.find(params[:id])
u = User.find(params[:attendings])
if params[:attendings]
e.attendings << u
e.save
respond_with e, api_template: :general_event, status: :no_content
end end
Last detail: I'm using Ember 1.0.0.rc6, jQuery 1.10.2 and gems acts_as_api and active_model_serializers in Rails.
So, to summarize, I want to get working just my PUT request in Ember to add Users attending to an Event through the has_many_and_belongs_to macro (using transactions, jQuery, or something else)