默认在Rails has_many关系上使用范围 [英] Use a scope by default on a Rails has_many relationship

问题描述

假设我有以下课程

class SolarSystem < ActiveRecord::Base
  has_many :planets
end

class Planet < ActiveRecord::Base
  scope :life_supporting, where('distance_from_sun > ?', 5).order('diameter ASC')
end

行星的范围为 life_supporting 和 SolarSystem has_many：planets 。我想定义我的has_many关系，以便当我向所有关联的行星询问 solar_system 时， life_supporting 范围会自动应用。本质上，我想要 solar_system.planets == solar_system.planets.life_supporting 。

Planet has a scope life_supporting and SolarSystem has_many :planets. I would like to define my has_many relationship so that when I ask a solar_system for all associated planets, the life_supporting scope is automatically applied. Essentially, I would like solar_system.planets == solar_system.planets.life_supporting.

• 我不是不要更改 scope：life_supporting 行星到

default_scope其​​中（'distance_from_sun>？'，5）.order （直径ASC）

我也想通过不必添加到 SolarSystem

I'd also like to prevent duplication by not having to add to SolarSystem

has_many：planets，：conditions => [’distance_from_sun> ？’，5]，：order => '直径ASC'

我想拥有类似的东西

has_many：planets，：with_scope => ：life_supporting

如@phoet所说，可能使用ActiveRecord无法实现默认范围。但是，我发现了两个潜在的解决方法。两者都防止重复。第一个虽然很长，但仍保持明显的可读性和透明性，第二个是输出清晰的辅助类型方法。

As @phoet said, it may not be possible to achieve a default scope using ActiveRecord. However, I have found two potential work arounds. Both prevent duplication. The first one, while long, maintains obvious readability and transparency, and the second one is a helper type method who's output is explicit.

class SolarSystem < ActiveRecord::Base
  has_many :planets, :conditions => Planet.life_supporting.where_values,
    :order => Planet.life_supporting.order_values
end

class Planet < ActiveRecord::Base
  scope :life_supporting, where('distance_from_sun > ?', 5).order('diameter ASC')
end

另一个更清洁的解决方案是将以下方法简单地添加到 SolarSystem

Another solution which is a lot cleaner is to simply add the following method to SolarSystem

def life_supporting_planets
  planets.life_supporting
end

并在任何要使用 solar_system.planets solar_system.life_supporting_planets >。

and to use solar_system.life_supporting_planets wherever you'd use solar_system.planets.

都没有回答这个问题，所以我只是把它们放在这里，以防其他任何人遇到这种情况。

Neither answers the question so I just put them here as work arounds should anyone else encounter this situation.

推荐答案

在Rails 4中，关联有一个可选的 scope 参数，该参数接受应用于 Relation 的lambda（请参见 ActiveRecord :: Associ ations :: ClassMethods ）

In Rails 4, Associations have an optional scope parameter that accepts a lambda that is applied to the Relation (cf. the doc for ActiveRecord::Associations::ClassMethods)

class SolarSystem < ActiveRecord::Base
  has_many :planets, -> { life_supporting }
end

class Planet < ActiveRecord::Base
  scope :life_supporting, -> { where('distance_from_sun > ?', 5).order('diameter ASC') }
end

在Rails 3中，有时可以通过使用 where_values_hash 处理更好的作用域来改进 where_values 解决方法条件由多个其中或哈希（此处不是这种情况）定义。

In Rails 3, the where_values workaround can sometimes be improved by using where_values_hash that handles better scopes where conditions are defined by multiple where or by a hash (not the case here).

has_many :planets, conditions: Planet.life_supporting.where_values_hash

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