4 files changed, 41 insertions, 1 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 41e05dd8c1..4d0c1a4178 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Serialize JSON attribute value `nil` as SQL `NULL`, not JSON `null`
+
+    *Trung Duc Tran*
+
 *   Return `true` from `update_attribute` when the value of the attribute
     to be updated is unchanged.
 
diff --git a/activerecord/lib/active_record/type/internal/abstract_json.rb b/activerecord/lib/active_record/type/internal/abstract_json.rb
index 513c938088..e19c5a14da 100644
--- a/activerecord/lib/active_record/type/internal/abstract_json.rb
+++ b/activerecord/lib/active_record/type/internal/abstract_json.rb
@@ -17,7 +17,11 @@ module ActiveRecord
         end
 
         def serialize(value)
-          ::ActiveSupport::JSON.encode(value)
+          if value.nil?
+            nil
+          else
+            ::ActiveSupport::JSON.encode(value)
+          end
         end
 
         def accessor
diff --git a/activerecord/test/cases/adapters/mysql2/json_test.rb b/activerecord/test/cases/adapters/mysql2/json_test.rb
index 6b7d259023..630cdb36a4 100644
--- a/activerecord/test/cases/adapters/mysql2/json_test.rb
+++ b/activerecord/test/cases/adapters/mysql2/json_test.rb
@@ -102,6 +102,22 @@ if ActiveRecord::Base.connection.supports_json?
       assert_equal(["v0", { "k1" => "v1" }], x.payload)
     end
 
+    def test_select_nil_json_after_create
+      json = JsonDataType.create(payload: nil)
+      x = JsonDataType.where(payload:nil).first
+      assert_equal(json, x)
+    end
+
+    def test_select_nil_json_after_update
+      json = JsonDataType.create(payload: "foo")
+      x = JsonDataType.where(payload:nil).first
+      assert_equal(nil, x)
+
+      json.update_attributes payload: nil
+      x = JsonDataType.where(payload:nil).first
+      assert_equal(json.reload, x)
+    end
+
     def test_rewrite_array_json_value
       @connection.execute %q|insert into json_data_type (payload) VALUES ('["v0",{"k1":"v1"}]')|
       x = JsonDataType.first
diff --git a/activerecord/test/cases/adapters/postgresql/json_test.rb b/activerecord/test/cases/adapters/postgresql/json_test.rb
index c74f70f251..273b2c1c7b 100644
--- a/activerecord/test/cases/adapters/postgresql/json_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/json_test.rb
@@ -113,6 +113,22 @@ module PostgresqlJSONSharedTestCases
     assert_equal(nil, x.payload)
   end
 
+  def test_select_nil_json_after_create
+    json = JsonDataType.create(payload: nil)
+    x = JsonDataType.where(payload:nil).first
+    assert_equal(json, x)
+  end
+
+  def test_select_nil_json_after_update
+    json = JsonDataType.create(payload: "foo")
+    x = JsonDataType.where(payload:nil).first
+    assert_equal(nil, x)
+
+    json.update_attributes payload: nil
+    x = JsonDataType.where(payload:nil).first
+    assert_equal(json.reload, x)
+  end
+
   def test_select_array_json_value
     @connection.execute %q|insert into json_data_type (payload) VALUES ('["v0",{"k1":"v1"}]')|
     x = JsonDataType.first