Merge pull request rails#16162 from chancancode/fix_json_coder

Fixed JSON coder when loading NULL from DB

activerecord/lib/active_record/coders/json.rb

@@ -6,7 +6,7 @@ def self.dump(obj)
       end
 
       def self.load(json)
-        ActiveSupport::JSON.decode(json)
+        ActiveSupport::JSON.decode(json) unless json.nil?
       end
     end
   end

activerecord/test/cases/serialized_attribute_test.rb

@@ -81,6 +81,24 @@ def test_serialized_json_attribute_returns_unserialized_value
     assert_equal(my_post.title, t.content["title"])
   end
 
+  # This is to ensure that the JSON coder is behaving the same way as 4.0, but
+  # we can consider changing this in the future.
+  def test_json_db_null
+    Topic.serialize :content, JSON
+
+    # Force a row to have a database NULL instead of a JSON "null"
+    id = Topic.connection.insert "INSERT INTO topics (content) VALUES(NULL)"
+    t = Topic.find(id)
+
+    assert_nil t.content
+
+    t.save!
+
+    # On 4.0, re-saving a row with a database NULL will turn that into a JSON
+    # "null"
+    assert_equal 1, Topic.where('content = "null"').count
+  end
+
   def test_serialized_attribute_declared_in_subclass
     hash = { 'important1' => 'value1', 'important2' => 'value2' }
     important_topic = ImportantTopic.create("important" => hash)