Question

In custom validation methods, why are the attributes passed as local variables instead of being accessible as instance variables?

I was expecting to use @title instead of title in the custom validation below, but @title is nil in the code below. title contains the actual data.

    attr_accessible :title
    validate :do_check_title

    def do_check_title
      title =~ /^Alice in/ || errors.add(:title, "Not Alice in Wonderland")
    end

Looking through the active_record/core.rb

   def initialize(attributes = nil)
     ...
     assign_attributes(attributes) if attributes
     ...
   end

And then in active_record/attribute_assignment.rb

   def _assign_attribute(k, v)
     public_send("#{k}=", v)

So I guess, the attributes should be available as instance variables in the validation function.

Why are they nil?

Answer 1

It doesn’t really matter if you access these from a validation or other instance methods. You can see what’s happening from the console (pry):

u = User.first
show-method u.first_name=

This gives you something like this:

generated_attribute_methods.module_eval("def #{attr_name}=(new_value); write_attribute('#{attr_name}', new_value); end", __FILE__, __LINE__)

Now if you take a look at the write_attribute method, you can see that it deletes attribute cache etc. and then assign to attributes and it’s just a hash.

u.attributes

So from now on, instead of the boring u.first_name = "foo", you can use this:

u.send :write_attribute, "first_name", "foo"

and it will do the same thing (3.2.10).