A tokamak view with nested collections https://github.com/caelum/tokamak

index.tokamak.rb

collection(@activities) do |collection|
  collection.members(:root => "activities") do |member, activity|

  # ...

  # COMMENTS
  if comments = @comments.try(:[], activity.id)
    member.members(:root => "comments", :collection => comments) do |member, comment|
      member.values { |comment_values|
        comment_values.timestamp comment.created_at.to_i
        comment_values.content comment.object.content
        comment_values.actor {
          comment_values.type    comment.actor_type.downcase
          comment_values.id      comment.actor_id
         }
      }
    end
  end

  # LIKES
  if likes = @likes.try(:[], activity.id)
    member.members(:root => "likes", :collection => likes) do |member, like|
      member.values { |like_values|
        like_values.timestamp like.created_at.to_i
        like_values.actor {
          like_values.type like.actor_type.downcase
          like_values.id   like.actor_id
        }
      }
    end
  end
  
  # ...
  
end

Result.json

{ "activities": 
  [  
    {

      "comments": [
        {
          "timestamp": 1296039053, 
          "content" : "Alias omnis tenetur assumenda eligendi nisi.",
          "actor": {
            "id": 2,
            "type": "user"
          }
        }
      ],
      
      "likes": [
        {
          "timestamp": 1296039053, 
          "actor": {
            "id": 2,
            "type": "user"
          }
        }
      ]

    }
  ]
}