Polymorphic association in Simpleform

Is there a way to display polymorphic association in simple_form view?

So far I've got below:

= simple_form_for(@chat, :html => { :class => "form-horizontal" }, :wrapper => "horizontal", defaults: { :input_html => { class: "form-control"}, label_html: { class: "col-lg-4" } } ) do |f|
    = f.error_notification

    .form-inputs
        = f.association :from_user
        = f.association :to_user
        = f.input :message
        = f.association :chattable

    .form-actions
        = f.button :submit

And below model:

class Chat < ActiveRecord::Base
    belongs_to :from_user, :foreign_key => 'from_user_id', class_name: 'User'
    belongs_to :to_user, :foreign_key => 'to_user_id', class_name: 'User'
    belongs_to :chattable, polymorphic: true

    validates :from_user, associated: true, presence: true
    validates :message, presence: true
end

This throws out below error:

uninitialized constant Chat::Chattable