无效参数：升级到 Rails 5 后出现 nil 错误答案

【问题标题】：invalid argument: nil error after upgrading to Rails 5无效参数：升级到 Rails 5 后出现 nil 错误
【发布时间】：2016-11-02 09:06:39
【问题描述】：

我升级到 Rails 5，启动服务器后，必须加载登录表单。那么就会出现这个错误：

ArgumentError - invalid argument: nil.:
  activerecord (5.0.0.1) lib/active_record/relation/spawn_methods.rb:36:in `merge'
  activerecord (5.0.0.1) lib/active_record/scoping/default.rb:119:in `block (2 levels) in build_default_scope'
  activerecord (5.0.0.1) lib/active_record/scoping/default.rb:117:in `block in build_default_scope'
  activerecord (5.0.0.1) lib/active_record/scoping/default.rb:141:in `evaluate_default_scope'
  activerecord (5.0.0.1) lib/active_record/scoping/default.rb:116:in `build_default_scope'
  activerecord (5.0.0.1) lib/active_record/scoping/named.rb:33:in `default_scoped'
  activerecord (5.0.0.1) lib/active_record/scoping/named.rb:28:in `all'
  activerecord (5.0.0.1) lib/active_record/scoping.rb:24:in `scope_attributes'
  activerecord (5.0.0.1) lib/active_record/scoping.rb:36:in `populate_with_current_scope_attributes'
  activerecord (5.0.0.1) lib/active_record/scoping.rb:43:in `initialize_internals_callback'
  activerecord (5.0.0.1) lib/active_record/core.rb:317:in `initialize'
  activerecord (5.0.0.1) lib/active_record/inheritance.rb:65:in `new'
  devise (4.2.0) app/controllers/devise/sessions_controller.rb:9:in `new'

我认为这似乎来自设计？搜索后我发现有人有同样的错误：

Turns it it was a gem which behind the scenes was attempting to do default_scope { nil }, looks like someone implemented protection against that (since it shouldn't really work)

我该怎么办？

【问题讨论】：

愚蠢的问题：你需要更新到 Rails 5 吗？您是否尝试更新设计 gem？
是的，我也升级了 Devise

标签： ruby-on-rails ruby-on-rails-5

【解决方案1】：

nil 不再作为merge 参数有效，因为该方法需要ActiveRecord_Relation 类的参数。

此问题与this PR on Rails 有关。这是rails 上的resolution commit。

为避免此错误：

在对关系调用之前检查merge 属性，并仅在它不为空时附加它。
或始终将有效的ActiveRecord_Relation 传递给merge，例如<ARModel>.all，而不是如果实际参数为nil，则为nil。

【讨论】：