ruby-on-rails - ActiveRecord - "first"中的 "scope"方法返回多条记录

Question

我是 Rails 新手，遇到了奇怪的问题。

这是一个代码示例:

class News < ActiveRecord::Base
  scope :pinned, -> { where(pinned: true).first }
end

如果有带有“pinned”标志的记录没有问题，当我调用 News.pinned返回一条记录。

我在日志中看到了这个查询:

SELECT `news`.* 
FROM `news` 
WHERE `news`.`pinned` = 1 
ORDER BY `news`.`id` ASC 
LIMIT 1

但是如果没有带有“固定”标志的记录，当我调用 News.pinned接下来的两个查询被执行:

SELECT `news`.* 
FROM `news` 
WHERE `news`.`pinned` = 1 
ORDER BY `news`.`id` ASC 
LIMIT 1

SELECT `news`.* FROM `news`

谢谢!

Answer 1

Friendy，这里是 ActiveRecord 的方法“范围”:

1   # File activerecord/lib/active_record/scoping/named.rb, line 145
2   def scope(name, body, &block)
3     extension = Module.new(&block) if block
4
5     # Check body.is_a?(Relation) to prevent the relation actually being
6     # loaded by respond_to?
7     if body.is_a?(Relation) || !body.respond_to?(:call)
8       ActiveSupport::Deprecation.warn(
9         "Using #scope without passing a callable object is deprecated. For "                "example `scope :red, where(color: 'red')` should be changed to "                "`    scope :red, -> { where(color: 'red') }`. There are numerous gotchas "                "in the former usage and it makes the implementation more complicated "                "and     buggy. (If you prefer, you can just define a class method named "                "`self.red`.)"
10      )
11    end
12
13    singleton_class.send(:define_method, name) do |*args|
14      if body.respond_to?(:call)
15        scope = all.scoping { body.call(*args) }
16        scope = scope.extending(extension) if extension
17      else
18        scope = body
19      end
20
21      scope || all
22    end
23  end

请注意第 21 行，如果范围为“nil”，则返回“all”。
在您的情况下，当您在第 15 行调用没有记录的“News.pinned”时，第一次咨询运行并且作用域接收“nil”，所以当它到达第 21 行时，因为作用域为“nil”，“all”正在运行第二个咨询并返回所有寄存器。

我已经通过删除第 21 行的“全部”来测试它是否覆盖了方法“范围”，而我只有一个查询

要绕过此使用:

class News < ActiveRecord::Base
  def self.pinned
    where(pinned: true).first
  end
end