Rails 5 命名范围具有多态性，UndefinedColumn

我正在尝试在我的模型中编写一个命名范围，以检索与另一个模型关联的所有实例。这是我需要范围来检索所有实例的模型EmailTemplate

class Answer < ApplicationRecord
  has_one :email_template, as: :templateable
  validates :text, presence: true

  scope :with_template, -> { where.not(email_template: nil) }
end

还有我的另一个模型

class EmailTemplate < ApplicationRecord
  belongs_to :templateable, polymorphic: true
  validates :pre, presence: true
  validates :post, presence: true
end

当我这样做时Answer.with_template，会出现此错误。

[3] pry(main)> Answer.with_template
   (6.6ms)  SELECT COUNT(*) FROM "answers" WHERE ("answers"."templateable" IS NOT NULL)
ActiveRecord::StatementInvalid: PG::UndefinedColumn: ERROR:  no existe la columna answers.templateable
LINE 1: SELECT COUNT(*) FROM "answers" WHERE ("answers"."templateabl...
                                              ^
: SELECT COUNT(*) FROM "answers" WHERE ("answers"."templateable" IS NOT NULL)
from /home/pedro/.rvm/gems/ruby-2.4.0/gems/activerecord-5.1.1/lib/active_record/connection_adapters/postgresql_adapter.rb:620:in `async_exec'

也试过这个，但不起作用：

scope :with_template, -> { where.not(templateable: nil) }