如何设置Rails Associations :: Preloader的预加载范围？ [英] How to set preload scope for Rails Associations::Preloader?

问题描述

我需要预加载具有复杂条件的模型关联。不，包含对我不起作用。

I need to preload associations of the model with complex conditions. NO, includes doesn't work for me. It generate wrong SQL for my task.

我看看 ActiveRecord :: Associations :: Preloader 和发现他接受了 preload_scope 参数：

I take a look to the ActiveRecord::Associations::Preloader and find that he take an preload_scope argument:

http://apidock.com/rails/v4.2.1/ActiveRecord/Associations/Preloader/preload

 def preload(records, associations, preload_scope = nil)
  # ...
 end

但是我找不到使用它的任何示例。在这种情况下， preload_scope 是什么？以及如何使用它来过滤关联？
谢谢！

But I can't find any example using it. What is preload_scope in this case? And how I can use it for filtering associations? Thanks!

推荐答案

ActiveRecord不会通过 #preloads ：
https ：//github.com/rails/rails/blob/0aefa97689d001ca9a98db76bbad1bbbb0e51c9c/activerecord/lib/active_record/relation.rb#L663 –您只能看到传递了记录和关联参数。

ActiveRecord doesn't expose this through #preloads on relation: https://github.com/rails/rails/blob/0aefa97689d001ca9a98db76bbad1bbbb0e51c9c/activerecord/lib/active_record/relation.rb#L663 – as you can see only records and associations parameters are passed.

您可以进入ActiveRecord内部并直接调用Preloader：

You can reach into ActiveRecord internals and call Preloader directly:

rows = Projects.all.to_a
ActiveRecord::Associations::Preloader.new.preload(rows, :current_task, Struct.new(:values, :bind_values).new({where: "active"}, []))

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