gistfile1.rb

# XXX more specific name.

class Person < ActiveRecord::Base
  exec_context :hoge, default: true do
    validate :foo, presence: true
    before_create :foobar
  end

  validate :bar, presence: true
end

person = Person.new
another = Person.new

Person.in_context(:hoge) do
  person.valid?  # => hoge
  another.valid? # => hoge
end

person = Person.in_context(:hoge).new
another = Person.new

person.save! #=> hoge
another.save! #=> not hoge