# 使用 has_and_belongs_to_many
  class User < ActiveRecord::Base
   has_and_belongs_to_many :groups
  end

  class Group < ActiveRecord::Base
   has_and_belongs_to_many :users
  end

  # 偏好方式 - using has_many :through
  class User < ActiveRecord::Base
   has_many :memberships
   has_many :groups, through: :memberships
  end

  class Membership < ActiveRecord::Base
   belongs_to :user
   belongs_to :group
  end

  class Group < ActiveRecord::Base
   has_many :memberships
   has_many :users, through: :memberships
  end

  # 差
  class Person
   validates :email, format: { with: /^([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})$/i }
  end

  # 好
  class EmailValidator < ActiveModel::EachValidator
   def validate_each(record, attribute, value)
    record.errors[attribute] << (options[:message] || 'is not a valid email') unless value =~ /^([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})$/i
   end
  end

  class Person
   validates :email, email: true
  end

 class User < ActiveRecord::Base
   scope :active, -> { where(active: true) }
   scope :inactive, -> { where(active: false) }

   scope :with_orders, -> { joins(:orders).select('distinct(users.id)') }
  end

  # 差劲
  class User < ActiveRecord::Base
   scope :active, where(active: true)
   scope :inactive, where(active: false)

   scope :with_orders, joins(:orders).select('distinct(users.id)')
  end

  # 好
  class User < ActiveRecord::Base
   scope :active, -> { where(active: true) }
   scope :inactive, -> { where(active: false) }

   scope :with_orders, -> { joins(:orders).select('distinct(users.id)') }
  end

  class User < ActiveRecord::Base
   def self.with_orders
    joins(:orders).select('distinct(users.id)')
   end
  end

    class Person
     def to_param
      "#{id} #{name}".parameterize
     end
    end

  Ruby
  class Person
  extend FriendlyId
  friendly_id :name, use: :slugged
  end