Convenience methods for Rails' flashes with I18n support

README

Given

    class Post < ActiveRecord::Base
        def to_s; title; end
    end

Then

    notice :create => :post
    # flash[:notice] = I18n.t :"post.create.notice", :scope => [ :flashes ], :post => @post.to_s
    error :create! => :post
    # flash.now[:error] = I18n.t :"post.create.error", ...
    
    error "let's just ignore I18n completely"
    # flash[:error] = "let's just ignore I18n completely"
    notice "cause we're jerks", true
    # flash.now[:notice] = "cause we're jerks"

There are multiple I18n keys to determine the message
a simple `notice :create => :post` would look in

    :"post.create.notice"
    :"create.notice"
    :create
    :notice

application_controller.rb

class ApplicationController < ActionController::Base
  ...
  
  def notice(*args)
    set_flash :notice, *args
  end
  
  def error(*args)
    set_flash :error, *args
  end
  
  def set_flash(level, *args)
    if args.first.is_a? String # bypass I18n completely
      message = args.shift
      immediate = !args.empty? # set_flash :error, "Something went wrong", (true || :now || truthy)
    end
    
    unless message
      if args.first.is_a? Hash # set_flash :notice, :create => :post
        i18n_key = args.first.first.first
        object = args.first.first.last
        if real = instance_variable_get("@#{object}")
          interpolation_options = { object => real.to_s }
        end
      else
        i18n_key = args.first # set_flash :notice, :create
      end
      
      if i18n_key.to_s.ends_with? '!' # set_flash :error, :create! => flash.now[error] = ...
        immediate = true
        i18n_key = i18n_key.to_s[0...-1]
      end
      
      defaults = returning [] do |a|
        a << "#{object}.#{i18n_key}.#{level}"
        a << "#{i18n_key}.#{level}"
        a << "#{i18n_key}"
        a << "#{level}"
      end.collect(&:to_sym)
      
      message = I18n.t defaults.shift, (interpolation_options || {}).update(:default => defaults, :scope => [ :flashes ])
    end
    
    immediate ? flash.now[level] = message : flash[level] = message
  end
end