如何为普通 Ruby 数组创建可组合范围

Question

我真正喜欢 Active Record 的一件事是它的命名范围，并且能够将范围链接在一起以构建富有表现力的查询。

用普通的 Ruby Enumerables/Arrays 实现这一点的类似方法是什么，最好不要以任何危险的方式对 Enumerable 或 Array 进行猴子修补？

例如：

### ActiveRecord Model
class User < ActiveRecord::Base
  scope :customers, -> { where(:role => 'customer') }
  scope :speaking, ->(lang) { where(:language => lang) }
end

# querying
User.customers.language('English')  # all English customers

### Plain-Ruby Array
module User
  class << self
    def customers
      users.select { |u| u[:role] == 'customer' }
    end

    def speaking(lang)
      users.select { |u| u[:language] == lang }
    end

    private

    def users
      [
        {:name => 'John', :language => 'English', :role => 'customer'},
        {:name => 'Jean', :language => 'French', :role => 'customer'},
        {:name => 'Hans', :language => 'German', :role => 'user'},
        {:name => 'Max', :language => 'English', :role => 'user'}
      ]
    end
  end
end

User.customers  # all customers
User.language('English')  # all English speakers
# how do I achieve something similar to User.customers.language('English') ...?

我知道我可以在模块内构建一个方法customers_with_language，但我正在寻找一种通用方法来解决这个问题，它可以使用任意数量的“范围”。

Answer 1

这是ScopableArray 的粗略实现，它继承了Array：

class ScopableArray < Array
  def method_missing(method_sym, *args)
    ScopableArray.new(select { |u| u[method_sym] == args[0] } )
  end
end

当这个类接收到一个它不能识别的方法时，它假设你想根据方法名称的字段和参数的值来过滤它：

users = ScopableArray.new([
    {:name => 'John', :language => 'English', :role => 'customer'},
    {:name => 'Jean', :language => 'French', :role => 'customer'},
    {:name => 'Hans', :language => 'German', :role => 'user'},
    {:name => 'Max', :language => 'English', :role => 'user'}
])

users.role('customer')
# => [{:name=>"John", :language=>"English", :role=>"customer"}, {:name=>"Jean", :language=>"French", :role=>"customer"}]
users.role('customer').language('English')
# => [{:name=>"John", :language=>"English", :role=>"customer"}]

---

您还可以查看ActiveRecord's implementation pattern 以获得更详细的方案，您可以通过传递名称和可调用块来定义范围，如下所示：

class ScopableArray2 < Array
  class << self
    def scope(name, body)
      unless body.respond_to?(:call)
        raise ArgumentError, 'The scope body needs to be callable.'
      end

      define_method(name) do |*args|
        dup.select! { |x| body.call(x, *args) }
      end
    end
  end
end

然后你可以这样做：

class Users < ScopableArray2
  scope :customers, ->(x) { x[:role] == 'customer' }
  scope :speaking, ->(x, lang) { x[:language] == lang }
end

users = Users.new([
        {:name => 'John', :language => 'English', :role => 'customer'},
        {:name => 'Jean', :language => 'French', :role => 'customer'},
        {:name => 'Hans', :language => 'German', :role => 'user'},
        {:name => 'Max', :language => 'English', :role => 'user'}
    ])

users.customers.speaking('English')
# => [{:name=>"John", :language=>"English", :role=>"customer"}]