Test that no datetime precision isn't truncated on assignment

Also, max precision (= 6) isn't regarded as unlimited precision for now.

activerecord/test/cases/date_time_precision_test.rb

@@ -29,7 +29,7 @@ def test_datetime_data_type_with_precision
 
     def test_datetime_precision_is_truncated_on_assignment
       @connection.create_table(:foos, force: true)
-      @connection.add_column :foos, :created_at,  :datetime, precision: 0
+      @connection.add_column :foos, :created_at, :datetime, precision: 0
       @connection.add_column :foos, :updated_at, :datetime, precision: 6
 
       time = ::Time.now.change(nsec: 123456789)
@@ -45,6 +45,26 @@ def test_datetime_precision_is_truncated_on_assignment
       assert_equal 123456000, foo.updated_at.nsec
     end
 
+    unless current_adapter?(:Mysql2Adapter)
+      def test_no_datetime_precision_isnt_truncated_on_assignment
+        @connection.create_table(:foos, force: true)
+        @connection.add_column :foos, :created_at, :datetime
+        @connection.add_column :foos, :updated_at, :datetime, precision: 6
+
+        time = ::Time.now.change(nsec: 123)
+        foo = Foo.new(created_at: time, updated_at: time)
+
+        assert_equal 123, foo.created_at.nsec
+        assert_equal 0, foo.updated_at.nsec
+
+        foo.save!
+        foo.reload
+
+        assert_equal 0, foo.created_at.nsec
+        assert_equal 0, foo.updated_at.nsec
+      end
+    end
+
     def test_timestamps_helper_with_custom_precision
       @connection.create_table(:foos, force: true) do |t|
         t.timestamps precision: 4

activerecord/test/cases/time_precision_test.rb

@@ -45,6 +45,26 @@ def test_time_precision_is_truncated_on_assignment
       assert_equal 123456000, foo.finish.nsec
     end
 
+    unless current_adapter?(:Mysql2Adapter)
+      def test_no_time_precision_isnt_truncated_on_assignment
+        @connection.create_table(:foos, force: true)
+        @connection.add_column :foos, :start,  :time
+        @connection.add_column :foos, :finish, :time, precision: 6
+
+        time = ::Time.now.change(nsec: 123)
+        foo = Foo.new(start: time, finish: time)
+
+        assert_equal 123, foo.start.nsec
+        assert_equal 0, foo.finish.nsec
+
+        foo.save!
+        foo.reload
+
+        assert_equal 0, foo.start.nsec
+        assert_equal 0, foo.finish.nsec
+      end
+    end
+
     def test_passing_precision_to_time_does_not_set_limit
       @connection.create_table(:foos, force: true) do |t|
         t.time :start,  precision: 3