Embe Data: How to update a record - rest

Question

I've got an record of my model 'item' via
store.find('item', 412).then(function(item){
...
});
The model looks like that:
item :{
title: "mySuperItem",
price: "45",
comments: [
23423,
656332
]
}
However, what I want is to change the comments IDs in the founded record (want to add a new one).
So I thought it would be fine to do something like that:
...
.then(function(item){
item.comments.push(234234);
item.save().then(...);
});
Unfortunately it's not working as expected. Ember throws curious exceptions!
Since I'm writing my Backend with the REST-API on my own (nodes.js/express.js) ember-data will send a simple OPTIONS-Call to the backend with the following headers:
Host: localhost:3000
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:42.0) Gecko/20100101 Firefox/42.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: de,en-US;q=0.7,en;q=0.3
Accept-Encoding: gzip, deflate
Origin: http://localhost:4200
Access-Control-Request-Method: PUT
Access-Control-Request-Headers: content-type,x-access-token
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache
My backend will return a answer which contains that header: Allow:"GET,HEAD,PUT" so it seems like my backend answered "Hey its okay to send PUT-Calls to that address". But ember prompts me to use CORS, but its not working since the method cannot be found in the CORS-Header 'Access-Control-Allow-Methods'.
How could that be? Maybe I misunderstand something
Edit for addition
As #torazaburo mentioned correctly, the model for Item has of course a hasMany-relation to "comments"-model. However here is the model for Item:
title: DS.attr('String'),
price: DS.attr('Number'),
comments: DS.hasMany('comment')
Also its now more clear that the problem is with the save()-Part. Its throws an unexpected "cross-origin" error as described above.
Any Help?

Answer 1

I think you should set Access-Control-Allow-Origin header also. For example:
Access-Control-Allow-Origin: *