didUpdateAttrs is not showing the correct oldAttrs. When I create a new associated object both the oldAttr and newAttr's both reflect the newly created Item in the value collection.

component.js

import Ember from 'ember';

export default Ember.Component.extend({
  didUpdateAttrs: function(attrs) {
    // attrs.newAttrs.ownedItems.value => 
    //   Class {canonicalState: Array[26], store: Class, relationship: ember$data$lib$system$relationships$state$has$many$$ManyRelationship, record: Class, currentState: Array[27]…}
    
    // attrs.oldAttrs.ownedItems.value =>
    //   Class {canonicalState: Array[26], store: Class, relationship: ember$data$lib$system$relationships$state$has$many$$ManyRelationship, record: Class, currentState: Array[27]…}
    
    // the oldAttrs and newAttrs both reflect the same ember data relationship after creating the new item in the purchase action
    // I would have thought the oldAttrs would reflect the OLD assocation and newAttrs would reflect the NEW/CURRENT assocation
    attrs.newAttrs.ownedItems.value == attrs.oldAttrs.ownedItems.value // true -- noo!!!
  }
})

controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  purchasedItems: function() {
    this.get('session.student.items')
  }).property('session.student.items.[]'),

  actions: {
    purchase: function (item) {
      this.store.createRecord('item', { code: item.id, student: this.get('session.student') }).save()
    }
  }
})

item.js

import DS from 'ember-data';

export default DS.Model.extend {
  code: DS.attr("string")
  student: DS.belongsTo("student") // belongs to student
}

student.js

import DS from 'ember-data';

export default DS.Model.extend {
  name: DS.attr("string")
  items: DS.hasMany("item") // has many items
}

template.hbs

{{caper-game purchase=(action 'purchase') ownedItems=purchasedItems}}

Did you start a case for this? I'm have the same problem...