Fix Hash#deep_merge bug and improve documentation — resolves #12738

Previously merging into a hash with a falsy value would not result in
the merge-block being called. The fix is simply to check for presence
of the key in the hash.

The documentation example for `deep_merge` now appropriately
demonstrates what a deep merge does.

1 files changed, 21 insertions, 11 deletions

diff --git a/activesupport/lib/active_support/core_ext/hash/deep_merge.rb b/activesupport/lib/active_support/core_ext/hash/deep_merge.rb
index dc86c92003..b53fa524c6 100644
--- a/activesupport/lib/active_support/core_ext/hash/deep_merge.rb
+++ b/activesupport/lib/active_support/core_ext/hash/deep_merge.rb
@@ -1,25 +1,35 @@
 class Hash
   # Returns a new hash with +self+ and +other_hash+ merged recursively.
   #
-  #   h1 = { x: { y: [4, 5, 6] }, z: [7, 8, 9] }
-  #   h2 = { x: { y: [7, 8, 9] }, z: 'xyz' }
+  #   h1 = { a: true, b: { c: [1,2,3] } }
+  #   h2 = { a: false, b: { x: [3,4,5] } }
   #
-  #   h1.deep_merge(h2) # => {x: {y: [7, 8, 9]}, z: "xyz"}
-  #   h2.deep_merge(h1) # => {x: {y: [4, 5, 6]}, z: [7, 8, 9]}
-  #   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"]}
+  #   h1.deep_merge(h2) #=> { a: false, b: { c: [1,2,3], x: [3,4,5] } }
+  #
+  # Like with Hash#merge in the standard library, a block can be provided
+  # to merge values:
+  #
+  #   h1 = { a: 100, b: 200, c: { c1: 100 } }
+  #   h2 = { b: 250, c: { c1: 200 } }
+  #   h1.deep_merge(h2) { |key, this_val, other_val| this_val + other_val }
+  #   # => { a: 100, b: 450, c: { c1: 300 } }
   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, &block)
-    other_hash.each_pair do |k,v|
-      tv = self[k]
-      if tv.is_a?(Hash) && v.is_a?(Hash)
-        self[k] = tv.deep_merge(v, &block)
+    other_hash.each_pair do |current_key, other_value|
+      this_value = self[current_key]
+
+      self[current_key] = if this_value.is_a?(Hash) && other_value.is_a?(Hash)
+        this_value.deep_merge(other_value, &block)
       else
-        self[k] = block && tv ? block.call(k, tv, v) : v
+        if block_given? && key?(current_key)
+          block.call(current_key, this_value, other_value)
+        else
+          other_value
+        end
       end
     end
     self