Allow passing block to deep_merge and deep_merge!

Hash#merge accepts block that you can use to customize how hash values
are merged. This change makes merge and deep_merge compatible.

activesupport/CHANGELOG.md

@@ -1,5 +1,8 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   An optional block can be passed to `Hash#deep_merge`. The block will be invoked for each duplicated key
+    and used to resolve the conflict. *Pranas Kiziela*
+
 *   ActiveSupport::Deprecation is now a class. It is possible to create an instance
     of deprecator. Backwards compatibility has been preserved.
 

activesupport/lib/active_support/core_ext/hash/deep_merge.rb

@@ -6,15 +6,21 @@ class Hash
   #
   #   h1.deep_merge(h2) #=> {:x => {:y => [7, 8, 9]}, :z => "xyz"}
   #   h2.deep_merge(h1) #=> {:x => {:y => [4, 5, 6]}, :z => [7, 8, 9]}
-  def deep_merge(other_hash)
-    dup.deep_merge!(other_hash)
+  #   h1.deep_merge(h2) { |key, old, new| Array.wrap(old) + Array.wrap(new) }
+  #   #=> {:x => {:y => [4, 5, 6, 7, 8, 9]}, :z => [7, 8, 9, "xyz"]}
+  def deep_merge(other_hash, &block)
+    dup.deep_merge!(other_hash, &block)
   end
 
   # Same as +deep_merge+, but modifies +self+.
-  def deep_merge!(other_hash)
+  def deep_merge!(other_hash, &block)
     other_hash.each_pair do |k,v|
       tv = self[k]
-      self[k] = tv.is_a?(Hash) && v.is_a?(Hash) ? tv.deep_merge(v) : v
+      if tv.is_a?(Hash) && v.is_a?(Hash)
+        self[k] = tv.deep_merge(v, &block)
+      else
+        self[k] = block && tv ? block.call(k, tv, v) : v
+      end
     end
     self
   end

activesupport/test/core_ext/hash_ext_test.rb

@@ -582,6 +582,16 @@ class HashExtTest < ActiveSupport::TestCase
     assert_equal expected, hash_1
   end
 
+  def test_deep_merge_with_block
+    hash_1 = { :a => "a", :b => "b", :c => { :c1 => "c1", :c2 => "c2", :c3 => { :d1 => "d1" } } }
+    hash_2 = { :a => 1, :c => { :c1 => 2, :c3 => { :d2 => "d2" } } }
+    expected = { :a => [:a, "a", 1], :b => "b", :c => { :c1 => [:c1, "c1", 2], :c2 => "c2", :c3 => { :d1 => "d1", :d2 => "d2" } } }
+    assert_equal(expected, hash_1.deep_merge(hash_2) { |k,o,n| [k, o, n] })
+
+    hash_1.deep_merge!(hash_2) { |k,o,n| [k, o, n] }
+    assert_equal expected, hash_1
+  end
+
   def test_deep_merge_on_indifferent_access
     hash_1 = HashWithIndifferentAccess.new({ :a => "a", :b => "b", :c => { :c1 => "c1", :c2 => "c2", :c3 => { :d1 => "d1" } } })
     hash_2 = HashWithIndifferentAccess.new({ :a => 1, :c => { :c1 => 2, :c3 => { :d2 => "d2" } } })