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, 10 insertions, 0 deletions

diff --git a/activesupport/test/core_ext/hash_ext_test.rb b/activesupport/test/core_ext/hash_ext_test.rb
index 1ee9fbf46e..0cbd119654 100644
--- a/activesupport/test/core_ext/hash_ext_test.rb
+++ b/activesupport/test/core_ext/hash_ext_test.rb
@@ -661,6 +661,16 @@ class HashExtTest < ActiveSupport::TestCase
     assert_equal expected, hash_1
   end
 
+  def test_deep_merge_with_falsey_values
+    hash_1 = { e: false }
+    hash_2 = { e: 'e' }
+    expected = { e: [:e, false, 'e'] }
+    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" } } })