Custom relations

It's now possible to create custom relations
e.g.

// models/User.js
this.has('has_posts_written', {
  query: function(store, parentRecords){
    const ids = parentRecords.map(r => r.id)
    const Post = store.Model('Post')
    // query all posts by user_id, group by user_id and count(distinct(id))
    return Post.totalCount().group('user_id').where({user_id: ids})
  },

  convert: function(parent, records){
    if(!records) return false
    // records => [{user_id: 1, count: 4}, {user_id: 2, count: 1}]
    // == the result of the above query!
    const result = records.find(r => r.user_id === parent.id)
    if(!result) return false
    return result.count > 0
  }
})

A custom relation could return anything. In the example above it'll return a boolean value.

Works with include() like any other relation:
User.include('has_posts_written')