dry_eraser

dry_eraser provides a dry run before you erase your models.

This gem is for people who think it's weird that Rails offers so many ways to validate models before you create and update them, but all it gives you is a before_destroy hook if before permanently destroying them.

Think of dry_eraser as adding validation for ActiveRecord#destroy. To that end, it defines dry_erase and dry_erasable? methods for your models that behave similarly to validates and valid?. This way, you won't have to remember how to register a before_destroy callback. Or that throw(:abort) is the magical incantation to cancel the callback chain. If you're suspicious of pulling in a dependency for something like this (and you should be), the fact its implementation is 50 lines soaking wet might put you at ease.

Here's how to use it.

Install

Add it to your Gemfile:

gem "dry_eraser"

That's it. Rails should load it automatically.

Usage

Whenever there's a situation in which you know you don't want a destroy operation to go through, you can specify it by calling the dry_erase class method on your model.

Let's take an example Whiteboard model. Suppose it has a boolean attribute called someone_wrote_do_not_erase_on_me and you want to be sure destroy operations are aborted when that attribute is true.

You could:

class Whiteboard < ActiveRecord::Base
  dry_erase :no_one_said_not_to_erase_it

  private

  def no_one_said_not_to_erase_it
    if someone_wrote_do_not_erase_on_me?
      errors.add(:someone_wrote_do_not_erase_on_me, "so I can't erase it")
    end
  end
end

This way, whenever someone_wrote_do_not_erase_on_me? is true, destroy will return false (just like save returns false when validations fail).

This, combined with the fact that dry_erase determines success based on the absence or presence of errors on the model instance will allow you to write code that branches on whether destroy succeeded, just like you would for save or update:

whiteboard = Whiteboard.create!(someone_wrote_do_not_erase_on_me: true)
if whiteboard.destroy
  flash[:notice] = "Whiteboard deleted!"
  redirect_to whiteboards_path
else
  flash[:error] = whiteboard.errors.full_messages
  render :show, status: :unprocessable_entity
end

If you don't want to call destroy to know whether a model is safe to destroy, you can also call dry_erasable? and it'll populate the errors object all the same:

whiteboard = Whiteboard.create!(someone_wrote_do_not_erase_on_me: true
whiteboard.dry_erasable?
=> false
whiteboard.errors.full_messages.first
=> "Someone wrote do not erase on me so I can't erase it"

Other stuff you can pass to dry_erase

The dry_erase method can take one or more of any of the following:

• A symbol or string name of an instance method on the model
• A class that has a no-arg constructor and a dry_erase(model) method
• An object that responds to a dry_erase(model) model
• An object (e.g. a proc or lambda) that responds to call(model)

You can see all of these uses in the gem's test fixture:

# You can put more than one on one line
dry_erase :must_have_content, AnnoyingCoworkerMessageEraser

# Or pass a lambda
dry_erase ->(model) { model.content == "🖍️" && model.errors.add(:base, "No crayon, c'mon!") }

# Or an instance of a class that requires static configuration
dry_erase ForeignKeyEraser.new(Classroom, :whiteboard)

And that's about it.