gistfile1.rb

class Post
  belongs_to :poster, polymorphic: true
end

class User
  has_many :posts, as: :poster
end

class Admin
  has_many :posts, as: :poster
end

class PostSerializer
  attributes :poster
  has_one :poster, polymorphic: true, embed: :ids, include: true
end

PostSerializer.new(Post.last).as_json 
# would be:
# {"post" => {poster: {id: 1, type: "admin"}},
#  "admins" => [{id: 1]}}

# want to make it:
# {"post" => {poster: {id: 1, type: "superuser"}},
#  "superusers" => [{id: 1]}}