对 Rails 的字段进行有条件的验证
class Exam < ApplicationRecord
validates :title, presence: true
validates :subject_id, presence: true, if: :no_exam_type?
belongs_to :subject
def no_exam_type?
!self.exam_type.present?
end
end
想实现对 exam 的 subject_id 进行验证
当 exam_type 字段为空时，subject_id 一定要存在
而当 exam_type 字段不为空时，subject_id 不用存在
但上面的写法一直不能成功。
通过 rails c 去测试 Exam.create!(title: "title test", exam_type: "test") 一直报错：
ActiveRecord::RecordInvalid: 验证失败: Subject 不能为空
给 belongs_to 加上 optional: true
因为:
4.1.2.11 :optional If you set the :optional option to true, then the presence of the associated object won’t be validated. By default, this option is set to false.
默认情况下， belongs_to 是一定要存在的，也即 optional: false
所以将代码修改如下，即可：
# exam.rb
class Exam < ApplicationRecord
validates :title, presence: true
validates :subject_id, presence: true, if: :no_exam_type?
belongs_to :subject, optional: true
def no_exam_type?
!self.exam_type.present?
end
end