main.rb

require "active_record"

# DB接続設定
ActiveRecord::Base.establish_connection(
  adapter:  "sqlite3",
  database:  ':memory:'
)

# スキーマの設定
class InitialSchema < ActiveRecord::Migration
  def self.up
    create_table :users do |t|
      t.string :first_name
      t.string :last_name
    end
  end

  def self.down
    drop_table :users
  end
end
InitialSchema.migrate(:up)

module FirstNameValidtionContext
  def self.included(klass)
    klass.instance_eval do |i|
      validates :first_name, presence: true
    end
  end
end

module LastNameValidtionContext
  def self.included(klass)
    klass.instance_eval do |i|
      validates :last_name, presence: true
    end
  end
end

class User < ActiveRecord::Base
end

class UserWithFirstNameValidationContext < User
  include FirstNameValidtionContext
end

class UserWithLastNameValidationContext < User
  include LastNameValidtionContext
end

u = UserWithFirstNameValidationContext.new
u.first_name = "";
u.last_name = "akasaki";
p u.validate # => false

u.first_name = "tsukika";
p u.validate # => true

u = UserWithLastNameValidationContext.new
u.first_name = "tsukika";
u.last_name = "";
p u.validate # => false

u.last_name = "akasaki";
p u.validate # => true