validates :terms_of_service, acceptance: true

acceptanceMap到默认的rails验证器AcceptanceValidator：
https://github.com/rails/rails/blob/main/activemodel/lib/active_model/validations/acceptance.rb
示例中列出了所有默认验证器：

• https：//www.example.com api.rubyonrails.org/classes/ActiveModel/Validations/ClassMethods.html#method-i-validates*

absence
acceptance
confirmation
exclusion
format
inclusion
length
numericality
presence

以及由ActiveRecord添加的其他验证器：

associated
uniqueness

https://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html
每个validaror的可用选项在这里的helper方法中有记录：
https://api.rubyonrails.org/classes/ActiveModel/Validations/HelperMethods.html

validates :terms_of_service, acceptance: true
# is the same as using a helper method
validates_acceptance_of :terms_of_service
# and both map to rails default `AcceptanceValidator`

您也可以使用自定义验证器：

validates :terms_of_service, terms: true # maps to `TermsValidator`

# because there is no TermsValidator class in rails, you have to define it
# class TermsValidator
# # TODO: see docs for examples of custom validators
# end

• https：//www.example.com guides.rubyonrails.org/active_record_validations.html#performing-custom-validations*