Ember fetch hasMany relations hook

I'm learning Ember (2.14) and currently working with Ember.Data. I need to fetch data from an API that returns an entity relations by its url on the server. Like so :

{
  "id": 12,
  "name" : "John",
  "friends": [
    "https://api/people/24",
    "https://api/people/55",
    "https://api/people/34",
  ]
}

I'm using the JSONSerializer, with normalize to parse fields of hasMany relations into a single id for Ember.Data to handle, like so

export default JSONSerializer.extend({
  normalize(typeClass, hash) {
      let fields = Ember.get(typeClass, 'fields');

      fields.forEach(function(field,index) {

        if(field === 'hasMany'){
          // extract id from every relation containing url with ids
          hash[index] = hash[index].map((value)=>{
            if(value.indexOf('my_api_domain') > -1){
              const id = value.substring(value.lastIndexOf('/')+1,value.length);
              return id.length > 0 ? id : value;
            }
            return value;
          });
        }
      });

      return this._super.apply(this, arguments);
    }
});

I don't know much of Ember yet, but it feels very messy, is there a cleaner way to do it ?