Handle relationships as well.

addon/utils/merge-deep.ts

@@ -39,10 +39,16 @@ function propertyIsOnObject(object: any, property: any) {
   }
 }
 
+// Ember Data models don't respond as expected to foo.hasOwnProperty, so we do a special check
+function hasEmberDataProperty(target: any, key: string, options: Options): Boolean {
+  let fields: Map<string, any> | null = options.safeGet(target, 'constructor.fields');
+
+  return fields instanceof Map && fields.has(key);
+}
+
 // Protects from prototype poisoning and unexpected merging up the prototype chain.
-function propertyIsUnsafe(target: any, key: string): Boolean {
-   // Special case for ember data model attributes.
-  if(target.constructor && target.constructor.attributes && target.constructor.attributes.has(key)) {
+function propertyIsUnsafe(target: any, key: string, options: Options): Boolean {
+  if(hasEmberDataProperty(target, key, options)) {
     return false;
   }
 
@@ -83,7 +89,7 @@ function buildPathToValue(source: any, options: Options, kv: Record<string, any>
 function mergeTargetAndSource(target: any, source: any, options: Options): any {
   getKeys(source).forEach(key => {
     // proto poisoning.  So can set by nested key path 'person.name'
-    if (propertyIsUnsafe(target, key)) {
+    if (propertyIsUnsafe(target, key, options)) {
       // if safeSet, we will find keys leading up to value and set
       if (options.safeSet) {
         const kv: Record<string, any> = buildPathToValue(source, options, {}, []);

tests/dummy/app/models/profile.js

@@ -1,7 +1,11 @@
 import DS from 'ember-data';
 import attr from 'ember-data/attr';
+import { belongsTo } from 'ember-data/relationships';
+
 
 export default DS.Model.extend({
   firstName: attr('string', { defaultValue: 'Bob' }),
   lastName: attr('string', { defaultValue: 'Ross' }),
+
+  pet: belongsTo('dog'),
 });

tests/integration/main-test.js

@@ -164,10 +164,12 @@ module('Integration | main', function(hooks) {
       assert.ok(true, "user save was called");
     };
     let profile = this.store.createRecord("profile", { save });
+    let pet = this.store.createRecord('dog')
     let profileChangeset = new Changeset(profile);
 
     profileChangeset.set("firstName", "bo");
     profileChangeset.set("lastName", "jackson");
+    profileChangeset.set('pet', pet)
 
     profileChangeset.save();
   });