title: How does rails active model serializer handle nested resource?
body: <p>Suppose that there are two tables projects and progresses, and one project has many progresses (ont-to-many):</p>
  
  <pre><code>class Project &lt; ApplicationRecord
    has_many :progresses
  end
  
  class Progress &lt; ApplicationRecord
    belongs_to :project
  end
  </code></pre>
  
  <p>I does define the ProjectSerializer like below:</p>
  
  <pre><code>class ProjectSerializer &lt; ApplicationSerializer
    attributes :title, :state, :tags # and so on
    attribute :lateset_progress
  
    def lateset_progress
      object.progresses.order(created_at: :desc).first
    end
  end
  </code></pre>
  
  <p>Also, I initialize my serializer in controller like this:</p>
  
  <pre><code>def show
    project = Project.find(params[:id])
    resource = ActiveModelSerializers::SerializableResource.new(
                 project, key_transform: :camel_lower, adapter: :json)
    render json: resource
  end
  </code></pre>
  
  <p>The problem is that the nested lastest_progress does not handled by serializer and render all attributes with underscore.</p>
  
  <p>The really response data is:</p>
  
  <pre><code>{
      "project": {
          "id": 1,
          "title": "a test title",
          "tags": [
              "tag-A",
              "tag-B"
          ],
          "latestProgress": {
              "id": 45,
              "details": "run run run",
              "project_id": 1,
              "created_at": "2018-07-10 04:14:59 UTC",
              "updated_at": "2018-07-10 04:14:59 UTC"
          }
      }
  }
  </code></pre>