Particularités

ember g helper include

import Ember from 'ember';

export function include(params) {
  const [items, value] = params;
  return items.indexOf(value) > -1;
}

export default Ember.Helper.helper(include);

Template :

<select class="form-control" multiple onChange={{action "selectService"}}>
  {{#each model.services as |service|}}
    <option value={{service.id}} selected={{include model.selectedIds service.id}}>{{service.title}}</option>
  {{/each}}
</select>

Classe dans Route handler :

const Services=Ember.Object.extend({
  selectedIds: [],

  selectedServices: Ember.computed('selectedIds.[]', function() {
    return this.get('selectedIds').map((id) => {
      return this.get('services').findBy('id', id);
    });
  })
});

Controller :

ember g controller select

import Controller from '@ember/controller';

export default Controller.extend({
  actions:{
  selectService:function(event) {
    const selectedIds = Ember.$(event.target).val();
    let model=this.get('model');
    model.set('selectedIds', selectedIds || []);
  }

}
});