title: How to detect and save relation change in Ember 2.13.0?
body: <p>I have documents list. Every document has client attribute defined by <strong>belongsTo</strong>.</p>
  
  <p>When user change client in one of documents i want to show in the counter how many documents are changed. And when user decide he will press "publish" button which will save documents client changes to api.</p>
  
  <p>DS.Model in ember 2.13 has parameters (<a href="https://emberjs.com/api/data/classes/DS.Model.html" rel="noreferrer">https://emberjs.com/api/data/classes/DS.Model.html</a>):</p>
  
  <p><strong>hasDirtyAttributes, 
  dirtyType</strong></p>
  
  <p>Both of them does not react on <strong>belongsTo/HasMany</strong> changes by Ember <strong>design</strong>. I saw many answers to this topic, but I do not see any <strong>isDirty()</strong> method for model in 2.13 documentation, nor any <strong>.send("becomeDirty")</strong> method to manually set document model in dirty status ? I also saw several plugins/mixins for older Ember versions.</p>
  
  <p>But my question is, how do Ember creators "advise/advice/best practice" to deal with this. Is there some basic way/manual solution that does not require any third party addon ? Like with maybe onchange observer for every relation in model?, or computed property with @each.dirtyType for child related models (or even set children will not be flagged as dirty itself?) ?</p>
  
  <p><strong>What is sandbox solution for this in Ember 2.13 ?</strong></p>