discourse/app/serializers/post_revision_serializer.rb

class PostRevisionSerializer < ApplicationSerializer
  attributes :post_id,
             :version,
             :revisions_count,
             :username,
             :display_username,
             :avatar_template,
             :created_at,
             :edit_reason,
             :body_changes,
             :title_changes,
             :category_changes,
             :user_changes,
             :wiki_changes,
             :post_type_changes

  def include_title_changes?
    object.has_topic_data?
  end

  def include_category_changes?
    object.has_topic_data?
  end

  def version
    object.number
  end

  def revisions_count
    object.post.version
  end

  def username
    user.username_lower
  end

  def display_username
    user.username
  end

  def avatar_template
    user.avatar_template
  end

  def edit_reason
    object.lookup("edit_reason", 1)
  end

  def user_changes
    obj = object.user_changes
    return unless obj
    # same as below - if stuff is messed up, default to system
    prev = obj[:previous_user] || Discourse.system_user
    new = obj[:current_user] || Discourse.system_user
    {
        previous: {
            username: prev.username_lower,
            display_username: prev.username,
            avatar_template: prev.avatar_template
        },
        current: {
            username: new.username_lower,
            display_username: new.username,
            avatar_template: new.avatar_template
        }
    }
  end

  def user
    # if stuff goes pear shape attribute to system
    object.user || Discourse.system_user
  end

end