deep_merge.rb

# adapted from:  http://github.com/railsmachine/shadow_puppet/blob/master/lib/shadow_puppet/core_ext.rb

require 'active_support/core_ext/class/attribute_accessors'
require 'active_support/core_ext/array'
require 'active_support/inflector'
require 'active_support/core_ext/class/inheritable_attributes'
require 'active_support/core_ext/duplicable'

class Hash #:nodoc:
  
  def deep_merge(other_hash)
    self.merge(other_hash) do |key, oldval, newval|
      oldval = oldval.to_hash if oldval.respond_to?(:to_hash)
      newval = newval.to_hash if newval.respond_to?(:to_hash)
      if oldval.is_a?(Hash) && newval.is_a?(Hash)
        oldval.deep_merge(newval)
      elsif oldval.is_a?(Array) && newval.is_a?(Array)
        oldval += newval
      else
        newval
      end
    end
  end
  
  def deep_merge!(other_hash)
    replace(deep_merge(other_hash))
  end
  
  def deep_symbolize_keys
    self.inject({}) { |result, (key, value)|
      value = value.deep_symbolize_keys if value.is_a?(Hash)
      result[(key.to_sym rescue key) || key] = value
      result
    }
  end
  
end

new_deep_merge

>> hash1 = {:name => "Tom", :with => {:no_fee => 1}, :without => {}, :conditions => {:town_id => 1}, :include => [:town, :state]}
=> {:include=>[:town, :state], :with=>{:no_fee=>1}, :without=>{}, :conditions=>{:town_id=>1}, :name=>"Tom"}

>> hash2 = {:name => "Thomas", :with => {}, :without => {:state_id => 1}, :conditions => {:state_id => 2}, :include => [:neighborhood]}
=> {:include=>[:neighborhood], :with=>{}, :without=>{:state_id=>1}, :conditions=>{:state_id=>2}, :name=>"Thomas"}

>> hash1.deep_merge!(hash2)
=> {:include=>[:town, :state, :neighborhood], :without=>{:state_id=>1}, :with=>{:no_fee=>1}, :conditions=>{:state_id=>2, :town_id=>1}, :name=>"Thomas"}

old_deep_merge_initial_thoughts

>> hash1 = {:name => "Tom", :with => {:no_fee => 1}, :without => {}, :conditions => {:town_id => 1}, :include => [:town, :state]}
=> {:include=>[:town, :state], :with=>{:no_fee=>1}, :without=>{}, :conditions=>{:town_id=>1}, :name=>"Tom"}

>> hash2 = {:name => "Thomas", :with => {}, :without => {:state_id => 1}, :conditions => {:state_id => 2}, :include => [:neighborhood]}
=> {:include=>[:neighborhood], :with=>{}, :without=>{:state_id=>1}, :conditions=>{:state_id=>2}, :name=>"Thomas"}

>> hash1.merge!(hash2) do |key,old_val,new_val| 
      if old_val.is_a?(Array)
        old_val += new_val
      elsif old_val.is_a?(String)
        new_val
      else
        old_val.merge!(new_val)
      end
    end

=> {:include=>[:town, :state, :neighborhood], :with=>{:no_fee=>1}, :without=>{:state_id=>1}, :conditions=>{:state_id=>2, :town_id=>1}, :name=>"Thomas"}