How to return all attributes of an object with Rails Serializer?

At least on 0.8.2 of ActiveModelSerializers you can use the following:

class GroupSerializer < ActiveModel::Serializer
  def attributes
    object.attributes.symbolize_keys
  end
end

Be carful with this though as it will add every attribute that your object has attached to it. You probably will want to put in some filtering logic on your serializer to prevent sensitive information from being shown (i.e., encrypted passwords, etc...)

This does not address associations, although with a little digging around you could probably implement something similar.

============================================================

UPDATE: 01/12/2016

On 0.10.x version of ActiveModelSerializers, attributes receives two arguments by default. I added *args to avoid exception:

class GroupSerializer < ActiveModel::Serializer
  def attributes(*args)
    object.attributes.symbolize_keys
  end
end