Allow Jbuilder instance in merge! (#485)

lib/jbuilder.rb

@@ -241,8 +241,15 @@ def attributes!
     @attributes
   end
 
-  # Merges hash or array into current builder.
-  def merge!(hash_or_array)
+  # Merges hash, array, or Jbuilder instance into current builder.
+  def merge!(obj)
+    hash_or_array =
+      if ::Jbuilder === obj
+        obj.attributes!
+      else
+        obj
+      end
+
     @attributes = _merge_values(@attributes, hash_or_array)
   end
 

test/jbuilder_test.rb

@@ -196,6 +196,18 @@ class JbuilderTest < ActiveSupport::TestCase
     assert_equal 'Pavel', result['author']['name']
   end
 
+  test 'support merge! method with Jbuilder instance' do
+    obj = jbuild do |json|
+      json.foo 'bar'
+    end
+
+    result = jbuild do |json|
+      json.merge! obj
+    end
+
+    assert_equal 'bar', result['foo']
+  end
+
   test 'blocks are additive via extract syntax' do
     person = Person.new('Pavel', 27)