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.