Is there a clean API for resetting instance variables on 'reload' in ActiveRecord?

Question

In an ActiveRecord::Base model, I can reset the state of the model to what it was when I got it from the database with reload, as long as the attribute I'm setting maps to a table column:

user = User.first
user.email #=> "[email protected]"
user.email = "[email protected]"
user.email #=> "[email protected]"
user.reload
user.email #=> "[email protected]"

But if I add a custom attribute, the only way I've found to have it act the same is like this:

class User < ActiveRecord::Base
  attr_accessor :user_agent

  def reload
    super
    self.user_agent = nil
    self
  end
end

My question is, is there some API to make non-database-column-attributes reset on reload? Something like:

class User < ActiveRecord::Base
  # this
  reloadable_attr_accessor :user_agent
  # or this
  def user_agent
    @user_agent
  end

  def user_agent=(value)
    set_instance_var_that_resets_on_reload("@user_agent", value)
  end
end

Does that exist in Rails somewhere?

Answer 1

Rework Jean-Do's answer slightly. It doesn't break default instance_variables and relations.

after_initialize do 
  @default_instance_variables = instance_variables
end

def reload(options = nil)
  super
  self.instance_variables.each do |ivar|
    if ivar == :'@default_instance_variables' || 
      @default_instance_variables.include?(ivar)
      next 
    end
    remove_instance_variable(ivar)
  end
  self
end

Answer 2

I took gayavat's answer and reworked it into my test_helper.rb file, because I didn't want to override the usual #reload method.

class ActiveRecord::Base
  attr_accessor :default_instance_variables
  after_initialize do 
    @default_instance_variables = instance_variables
  end
end
def reload_ivars(obj)
  obj.reload
  obj.instance_variables.each do |ivar|
    if ivar == :'@default_instance_variables' || 
     obj.default_instance_variables.include?(ivar)
      next 
    end
    obj.send(:remove_instance_variable, ivar)
  end
end

When I need to reload something in a test I just call reload_ivars(object).

Answer 3

I've reworked gavayat's answer slightly. I've made three changes:

• Using a class variable to store default_instance_variables makes a bit more sense here (unless you're conditionally instantiating instance variables in your constructor, but you don't want to do that). This way you don't need to check for default_instance_variables itself inside reload.
• I've moved the super call to the end of the function. This ensures that this method returns whatever ActiveRecord::Base.reload returns, and gets rid of a line of code.
• The options parameter is actually passed into the super call.

after_initialize do 
  @@default_instance_variables ||= instance_variables
end

def reload(options = nil)
  self.instance_variables.each do |ivar|
    if @@default_instance_variables.include?(ivar)
      next 
    end
    remove_instance_variable(ivar)
  end
  super(options)
end