Fix null value on bytestring columns

clickhouse_driver/columns/stringcolumn.py

@@ -10,12 +10,13 @@
 class String(Column):
     ch_type = 'String'
     py_types = compat.string_types
+    null_value = ''
 
     # TODO: pass user encoding here
 
     def prepare_null(self, value):
         if self.nullable and value is None:
-            return '', True
+            return self.null_value, True
 
         else:
             return value, False
@@ -34,6 +35,7 @@ def read_items(self, n_items, buf):
 
 class ByteString(String):
     py_types = (bytearray, bytes)
+    null_value = b''
 
     def write_items(self, items, buf):
         for value in items:
@@ -93,6 +95,7 @@ def write_items(self, items, buf):
 
 class ByteFixedString(FixedString):
     py_types = (bytearray, bytes)
+    null_value = b''
 
     def read_items(self, n_items, buf):
         length = self.length

tests/columns/test_fixedstring.py

@@ -129,3 +129,25 @@ def test_not_decoded(self):
             )
             self.assertIsInstance(inserted[0][0], bytes)
             self.assertIsInstance(inserted[1][0], bytes)
+
+    def test_nullable(self):
+        with self.create_table('a Nullable(FixedString(10))'):
+            data = [
+                (None, ),
+                (b'test\x00\x00\x00\x00\x00\x00', ),
+                (None, ),
+                (b'nullable\x00\x00', )
+            ]
+            self.client.execute(
+                'INSERT INTO test (a) VALUES', data
+            )
+
+            query = 'SELECT * FROM test'
+            inserted = self.emit_cli(query)
+            self.assertEqual(
+                inserted,
+                '\\N\ntest\\0\\0\\0\\0\\0\\0\n\\N\nnullable\\0\\0\n'
+            )
+
+            inserted = self.client.execute(query)
+            self.assertEqual(inserted, data)

tests/columns/test_string.py

@@ -103,3 +103,17 @@ def test_not_decoded_bytearray_expected(self):
                 )
 
                 self.assertIn('Column a', str(e.exception))
+
+    def test_nullable(self):
+        with self.create_table('a Nullable(String)'):
+            data = [(None, ), (b'test', ), (None, ), (b'nullable', )]
+            self.client.execute(
+                'INSERT INTO test (a) VALUES', data
+            )
+
+            query = 'SELECT * FROM test'
+            inserted = self.emit_cli(query)
+            self.assertEqual(inserted, '\\N\ntest\n\\N\nnullable\n')
+
+            inserted = self.client.execute(query)
+            self.assertEqual(inserted, data)