get length of hasMany relationship without triggering fetch

Question

model:

DS.Model.extend({
  title: DS.attr('string'),
  body: DS.attr('string'),
  comments: DS.hasMany('comment', { async: true} ),

  hasComments: Ember.computed.gt('comments.length', 0)
});

payload:

{ 
  "id": "abcdefg",
  "title": "some cats are cool",
  "body": "",
  "comments: ["100", "101", "102"]
}

But the hasComments computed property triggers a fetch for each comment individually.. I don't want this :D

I know this works (avoids the fetch), but reaches into private API:

hasComments: Ember.computed.gt('data.comments.length', 0)

ember.js 1.8.1

ember-data 1.0.0-beta.11

Any other recommendations on achieving a computed property based off of the length

Answer 1

As of Ember Data 2.5 (mentioned in release notes) there is a feature called ds-references which provides a way to check if if a hasMany relationship contains items or not without triggering a fetch for those items:

export default Model.extend({
  title: DS.attr('string'),
  body: DS.attr('string'),
  comments: DS.hasMany('comment', { async: true} ),

  hasComments: Ember.computed('comments', function() {
    return this.hasMany('comments').ids().length > 0;
  })
});

See this working in an Ember Twiddle

The ds-references feature implements the references API as described in RFC 57. References is a low level API to perform meta-operations on records, has-many relationships and belongs-to relationships:

• get the current local data synchronously without triggering a fetch or producing a promise
• notify the store that a fetch for a given record has begun, and provide a promise for its result
• similarly, notify a record that a fetch for a given relationship has begun, and provide a promise for its result
• retrieve server-provided metadata about a record or relationship

Source: Ember forum post and Ember Data 2.5 release blog post