Rails: Missing polymorphic: true Association Option

I've inherited a database and am trying to make sense of the associations.

In my Agent model:

class Agent < ActiveRecord::Base
  has_many :created_events, :class_name => "Event", :as => :creator
  has_many :created_tasks, :class_name => "Task", :as => :creator
  ....
end

In my Event model:

class Event < ActiveRecord::Base
  belongs_to :creator, :class_name => 'Agent'
  ....
end

In my Task model: class Task < ActiveRecord::Base belongs_to :creator, :class_name => 'Agent' ... end

What type of association is this? What is it doing?

In my research, all of the examples either use polymorphic: :true - for polymorphic associations, or foreign_key: - to set the foreign key directly. Neither of these association options are present in any of my models.