Create child association from controller - ruby-on-rails

Question

I have 2 associated models and am trying to create the child from the parent's controller:
class Purchase < ActiveRecord::Base
has_many :comments, as: :commentable, dependent: :destroy
accepts_nested_attributes_for :comments, reject_if: :all_blank, allow_destroy: true
end
class Comment < ActiveRecord::Base
belongs_to :employee
belongs_to :commentable, polymorphic: true
validates_presence_of :body, :employee_id, :commentable_id, :commentable_type
end
In my controller I accept the params for comment:
def create
#purchase = Purchase.new(purchase_params)
if #purchase[:discount] && #purchase[:discount] > 0
#purchase[:discount] = #purchase[:discount].round(3)
end
if #purchase.save
render json: #purchase, status: :created
else
p "errors", #purchase.errors
render json: #purchase.errors.full_messages, status: :unprocessable_entity
end
end
private
def purchase_params
params.require(:purchase).permit(
:sale_type, :weeks, :weekly_cost, :date, :discount, :client_id, :id, :created_at, :updated_at, :employee_id, :consultation_id, :program_id, :purchaseable_type, :purchaseable_id,
comments_attributes: [:body, :id, :employee_id, :commentable_type]
)
end
When I attempt to create the purchase with the following params, I get the error:
purchase = {
client_id: client.id,
weeks: 5,
weekly_cost: 295,
comments_attributes: [{
employee_id: employee.id,
body: "This is a test",
commentable_type: "Purchase"
}]
}
#<ActiveModel::Errors:0x00007fb0ef3849e0 #base=#<Purchase id: nil, weeks: 5, weekly_cost: 295.0, created_at: nil, updated_at: nil, client_id: 18, discount: nil, employee_id: nil, comment: nil, referral_used: false, deleted_at: nil, date: nil, sale_type: nil, purchaseable_type: nil, purchaseable_id: nil, program_id: nil>, #messages={:"comments.commentable_id"=>["can't be blank"]}, #details={:"comments.commentable_id"=>[{:error=>:blank}]}>
There's no way to know the commentable_id before saving the purchase. What am I missing?