ruby-on-rails - Rails 的 QueryObject - 如何封装 ActiveRecord 上的一组操作

Question

如何封装一组操作(其中大部分本质上是 .where(...))并将其应用于不同的模型，以使某些模型可能无法实现某些操作并应返回空集合。 (非必要代码跳过)

我设计的(但不满意):

class ActivityFinder

  def self.find(query, limit = nil)
    activities = get_activities(query)
    activities = activities.sort_by(&:created_at).reverse // some-kind of merge-sort
    activities = activities.slice(0, limit) if limit.present?
    activities
  end

  private

  def self.get_activities(query)
    activities = []
    activities += query.apply(ModelA)
    activities += query.apply(ModelB)
    activities += query.apply(ModelC)
    activities
  end
end

class ActivityQuery

  def created_before(time)
    @created_before = time
    self
  end

  def created_after(time)
    @created_after = time
    self
  end

  def apply(activity)
    activity = activity.where("#{activity.table_name}.created_at < ?", @created_before) if @created_before.present?
    activity = activity.where("#{activity.table_name}.created_at >= ?", @created_after) if @created_after.present?
    // more operations, not all of them are suported by ModelC
  rescue NoMethodError
    return []
  end
end

用法

query = ActivityQuery.new.created_before(last_time).with_hash(...)
activities = ActivityFinder.find(query)

我不喜欢的:

NoMethodError

的救援

如果不同的模型有不同的字段名称，则必须作为 case 处理查询中的语句，以这种方式将查询对象与每个模型耦合

所以我正在寻找更好实现的建议

更新

问题是我想传递从 ActiveModel 获得的任何对象，例如ActiveRecord::Relation，所以我不能只用方法定义一个模块(并在需要时覆盖)并将它包含在我正在使用的模型中。问题更多的是为干净的设计指明正确的方向，而不是关于实现细节，我会以某种方式弄清楚

Answer 1

为避免紧密耦合，请将模型特定代码放入模型中:

class ModelA < ActiveRecord::Base
  scope :created_before, ->(timestamp) { where("created_at < ?", timestamp) }
  scope :created_after, ->(timestamp) { where("created_at >= ?", timestamp) }
  scope :with_hash, ->(hash) { ... }
end

class ModelB < ActiveRecord::Base
  scope :created_before, ->(timestamp) { where("other_column < ?", timestamp) }
  scope :created_after, ->(timestamp) { where("other_column >= ?", timestamp) }
  scope :with_hash, where('false') # empty, chain-able collection
end

现在你有了一个一致的接口(interface)，你可以针对它进行编程:

class ActivityQuery

  def apply(activity)
    activity = activity.scoped
    activity = activity.created_before(@created_before) if @created_before.present?
    activity = activity.created_after(@created_after) if @created_after.present?
    activity = activity.with_hash(@hash) if @hash.present?
    activity
  end

end

不用抢救 NoMethodError并且不再摆弄模型的实现细节。