In rails, it is possible to specify when the validation should happen. sometimes you may want to introduce new validation or skip some. Rails on option helps us achieve this.

class Post < ApplicationRecord
 validates :body, presence: true
 validates :title, uniqueness: true, on: :create 
 validates :published, exclusion: [nil], on: :update 
end

From the above validations:

• it’s not possible to create or update a post without a body
• it will be possible to update a post with a duplicate title
• it will be possible to create a record with published nil

Apart from the commonly used on: :create and on: :update rails allow us to provide custom contexts.

class Post < ApplicationRecord
 validates :title, presence: true 
 validates :published_at, presence: true, on: :publish
end

From the above example, we are validating published_at presence on: :publish. Custom contexts are triggered explicitly by passing the name of the context to valid? invalid? or save

post = Post.new(title: "Rails validations")
post.valid? # true 
post.valid?(context: :publish) # false 
post.save(context: :publish) # false