ruby-on-railsactiverecordoverridingmethod-missingdynamic-attributes

Using method missing in Rails


I have a model with several date attributes. I'd like to be able to set and get the values as strings. I over-rode one of the methods (bill_date) like so:

  def bill_date_human
    date = self.bill_date || Date.today
    date.strftime('%b %d, %Y')
  end
  def bill_date_human=(date_string)
    self.bill_date = Date.strptime(date_string, '%b %d, %Y')
  end

This performs great for my needs, but I want to do the same thing for several other date attributes... how would I take advantage of method missing so that any date attribute can be set/get like so?


Solution

  • As you already know signature of desired methods it might be better to define them instead of using method_missing. You can do it like that (inside you class definition):

    [:bill_date, :registration_date, :some_other_date].each do |attr|
      define_method("#{attr}_human") do
        (send(attr) || Date.today).strftime('%b %d, %Y')
      end   
    
      define_method("#{attr}_human=") do |date_string|
        self.send "#{attr}=", Date.strptime(date_string, '%b %d, %Y')
      end
    end
    

    If listing all date attributes is not a problem this approach is better as you are dealing with regular methods instead of some magic inside method_missing.

    If you want to apply that to all attributes that have names ending with _date you can retrieve them like that (inside your class definition):

    column_names.grep(/_date$/)
    

    And here's method_missing solution (not tested, though the previous one is not tested either):

    def method_missing(method_name, *args, &block)
      # delegate to superclass if you're not handling that method_name
      return super unless /^(.*)_date(=?)/ =~ method_name
    
      # after match we have attribute name in $1 captured group and '' or '=' in $2
      if $2.blank?
        (send($1) || Date.today).strftime('%b %d, %Y')
      else
        self.send "#{$1}=", Date.strptime(args[0], '%b %d, %Y')
      end
    end
    

    In addition it's nice to override respond_to? method and return true for method names, that you handle inside method_missing (in 1.9 you should override respond_to_missing? instead).