add relations

app/serializers/follow_serializer.rb

 class FollowSerializer < ActiveModel::Serializer
   cache key: 'follow', expires_in: 24.hours
 
-  def follower
-    object.user
-  end
+  attributes :id
 
-  attributes :id, :follower, :followable
+  belongs_to :followable
+  has_many :user
 end