Accommodate value of None for ON DUPLICATE KEY UPDATE

Fixed bug where MySQL ON DUPLICATE KEY UPDATE would not accommodate setting
a column to the value NULL.  Pull request courtesy Lukáš Banič.

Fixes: #4715
Closes: #4716
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4716
Pull-request-sha: bfad6e6bdfb7d6dd5176deaf30d875f3d0f15e06

Change-Id: Ia8831cc171d131bf3824be8db4fd1d231231bba3

1 files changed, 6 insertions, 6 deletions

diff --git a/lib/sqlalchemy/dialects/mysql/base.py b/lib/sqlalchemy/dialects/mysql/base.py
index ad5ab288c..9aa80a5f4 100644
--- a/lib/sqlalchemy/dialects/mysql/base.py
+++ b/lib/sqlalchemy/dialects/mysql/base.py
@@ -1232,15 +1232,15 @@ class MySQLCompiler(compiler.SQLCompiler):
                 c for c in self.statement.table.c if c.key not in ordered_keys
             ]
         else:
-            # traverse in table column order
             cols = self.statement.table.c
 
         clauses = []
-        for column in cols:
-            val = on_duplicate.update.get(column.key)
-            if val is None:
-                continue
-            elif coercions._is_literal(val):
+        # traverses through all table columns to preserve table column order
+        for column in (col for col in cols if col.key in on_duplicate.update):
+
+            val = on_duplicate.update[column.key]
+
+            if coercions._is_literal(val):
                 val = elements.BindParameter(None, val, type_=column.type)
                 value_text = self.process(val.self_group(), use_schema=False)
             elif isinstance(val, elements.BindParameter) and val.type._isnull: