commentable-mixin

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  model: null,
  text: '',
  
  actions: {
    addComment() {
      let text = this.get('text');
      let comment = this.store.createRecord('comment', { text });
      
      // this line throws the error
      comment.set('parent', this.get('model'));  
    }
  }
  

});

mixins.commentable.js

import Ember from 'ember';
import { belongsTo, hasMany } from "ember-data/relationships";

export default Ember.Mixin.create({
  comments: hasMany('comment')
});

models.blog-post.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo, hasMany } from "ember-data/relationships";
import Commentable from '../mixins/commentable';

export default Model.extend(Commentable, {
});

models.comment.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo, hasMany } from "ember-data/relationships";

export default Model.extend({
  parent: belongsTo('commentable', { polymorphic: true }),
  text: attr('string')
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
    model() {
    return this.store.createRecord('blog-post');
  }
});

templates.application.hbs

Commentable as a mixin

<br />
<br />

Model: {{model}}

<br />
<br />

Blog post comments ({{model.comments.length}})

{{#each model.comments as |comment|}}
  <div>
    text: {{comment.text}}
  </div>
{{/each}}

<br />
<br />

<form {{action "addComment"  on="submit"}}>

{{input value=text placeholder="Add new comment"}}

<input type="submit" />
</form>

twiddle.json

{
  "version": "0.10.4",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.7.0",
    "ember-data": "2.7.0",
    "ember-template-compiler": "2.7.0"
  },
  "addons": {}
}