Adding an if method to ActiveRecord::Relation

I often find myself chaining ActiveRecord scopes, like this:

q = Article
  .not_deleted
  .published_within(start_date, end_date)
q = q.with_tags(tags) if tags.any?

I wish there were a nicer way to put in the conditional scopes, without assigning to a temporary variable. What I really want is to write this:

Article
  .not_deleted
  .published_within(start_date, end_date)
  .if(tags.any?) { with_tags(tags) }

Wouldn’t that be nicer?

Many years ago I brought this up on the pdxruby mailing list, and no one seemed very interested, but I’ve always wanted it in my projects.

Here is a naïve implementation, which just for simplicity I’ll add to Object instead of ActiveRecord::Relation (where it really belongs):

class Object

  def if(condition, &block)
    condition ? instance_eval(&block) : self
  end

end

5.if(true)  { self + 7 }    # equals 12
5.if(false) { self + 7 }    # equals 5

That almost works! The problem is that inside the block, things don’t really act like a closure. If we used this implementation, our example above would give a NameError about not finding a tags method or local variable. That’s because everything inside the block is evaluated with self set to the ActiveRecord::Relation instance.

Fortunately there is a way to fix it! Before calling the block, we can save the outside self, and then we can use method_missing to delegate any failures to there.

There is a nice writeup of this “cloaking” trick if you want more details. But if you read that article, perhaps you will notice it is not thread-safe, because it temporarily adds a method to the class, and two threads could stomp on each other if they did that at the same time.

This approach was in Rails ActiveSupport for a while as Proc#bind. They even fixed the multi-threading problem (more or less . . .) by generating a different method name every time. Unfortunately that created a new problem: since define_method takes a symbol, this creates more and more symbols, which in Ruby are never garbage collected! Effectively this is a memory leak. Eventually the Rails team deprecated it.

But we can still add something similar that doesn’t leak memory and is thread-safe. We just have to protect the brief moment when we define the method and then remove it, which is simple with a Mutex. In theory taking a lock adds some overhead, and possibly contention, but we don’t expect this to be a “hot spot”, so in practice the contention should be zero and the overhead trivial.

And here is our implementation (not on Object any more):

module ActiveRecord
  class Relation

    CLOAKER_MUTEX = Mutex.new

    def if(condition, &block)
      if condition
        meth = self.class.class_eval do
          CLOAKER_MUTEX.synchronize do
            define_method :cloaker_, &block
            meth = instance_method :cloaker_
            remove_method :cloaker_
            meth
          end
        end
        with_previous_context(block.binding) { meth.bind(self).call }
      else
        self
      end
    end

    def with_previous_context(binding, &block)
      @previous_context = binding.eval('self')
      result = block.call
      @previous_context = nil
      result
    end

    def method_missing(method, *args, &block)
      super
    rescue NameError => e
      if @previous_context
        @previous_context.send(method, *args, &block)
      else
        raise e
      end
    end
  end
end

Put that in config/initializers and try it out!

UPDATE: I’ve been asked for a license for this code, so I’m releasing it under the MIT license:

Copyright © 2018–2022 Paul Jungwirth

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.