Rails4:在has_many模型验证器上使用has_many保存对象失败

我正试图一次保存两个模型(一对多)。我的代码如下所示:

@submission = Submission.new(submission_params)

@submission_asset = @submission.attachments.new(submission_asset_params)
@submission_asset.attachment_type = 'submission_asset'

if @submission.save
  # render or redirect here
else
  @submission.errors
end

但当我运行这个时,我会出现这个错误 @messages={:attachments=>["is invalid"]} 。我想这是因为我的依恋模型有这个:

# Attachment model snippet
validates :attachable_id, :attachable_type, presence: true

但这是为了确保它连接到 Submission 。但当我删除或注释掉验证时,它会起作用,并保存两个模型和关联。

我该如何保存?

编辑

class Submission < ActiveRecord::Base
  has_many :attachments, as: :attachable, dependent: :destroy
end

class Attachment < ActiveRecord::Base
  belongs_to :attachable, polymorphic: true

  validates :attachable_id, :attachable_type, presence: true
end