Why aren't Rails model attributes accessible using symbols instead of strings?

Question

I need to compare some Rails (2.3.11) model attribute values before and after a database update, so I start by finding my record and saving the existing attribute values in a hash, as follows:

id = params[:id]
work_effort = WorkEffort.find(id)

ancestor_rollup_fields = {
    :scheduled_completion_date => work_effort.scheduled_completion_date
}

work_effort.update_attributes(params.except(:controller, :action))
#etcetera

Note I am adhering to the "best practice" of using a symbol for a hash key.

Then I have a method that takes the model and the hash to determine possible additional steps to take if the values from the hash and the model attributes don't match. To determine this I tried to get at the model attribute value in an each loop but I was getting nil at first:

def rollup_ancestor_updates(work_effort, ancestor_rollup_fields)
    ancestor_rollup_fields.each do |key, value|
        model_val = work_effort.attributes[key] #nil
        #etcetera

In debugging the above I noticed that hard-coding a string as a key:

work_effort.attribute['scheduled_completion_date']

Returned the desired value. So then in my each block I tried the following and it worked:

model_val = work_effort.attributes[key.to_s]

Is there a different way to do this? To me, with just 3 months Ruby/Rails experience, it's confusing to use symbols as hash keys as is the prescribed best practice, but then have to call .to_s on the symbol to get at a model attribute. Has anybody else experienced this, worked around this, been confused by this too? Thanks in advance

Answer 1

The Hash returned when you call #attributes on a AR instance has string keys, which is why a symbol as an index into the hash doesn't work in your case. There is a subclass of Hash called HashWithIndifferentAccess which automatically converts symbol indexes into strings.

Quite often in Rails you'll encounter HashWithIndifferentAccess instances. A perfect example is the params variable you access in your controller and view code.

Try using work_effort.attributes.with_indifferent_access[key]

Really it is just doing the same thing that you are, but it does it behind the scenes.

Answer 2

You can overwrite the attributes method with your own.

Open your WorkEffort class

class WorkEffort
  def attributes
    super.symbolize_keys
  end
end

then when you call work_effort.attributes, you will have symbolized keys in the hash.

Answer 3

You may want to use : stringify_keys! which is extensively used all over Rails code.

def rollup_ancestor_updates(work_effort, ancestor_rollup_fields)
  ancestor_rollup_fields.stringify_keys!.each do |key, value|
    model_val = work_effort.attributes[key]
  end
  #....
end

Answer 4

All of your Rails models can have symbolized attribute names:

class ApplicationRecord < ActiveRecord::Base
  # ...

  class << self
    def attribute_names(symbols = false)
      return self.attribute_names.map(&:to_sym) if symbols

      super()
    end
  end
end

Then you can call MyModel.attribute_names(symbols = true) and get back [:id, :my_attribute, :updated_at, :created_at, ...] while still allowing attribute_names to return strings.

Note that you need the parentheses after super to avoid an argument error with the originally defined function.