Merge "Accommodate value of None for ON DUPLICATE KEY UPDATE"

doc/build/changelog/unreleased_13/4715.rst

@@ -0,0 +1,6 @@
+.. change::
+    :tags: bug, mysql
+    :tickets: 4715
+
+    Fixed bug where MySQL ON DUPLICATE KEY UPDATE would not accommodate setting
+    a column to the value NULL.  Pull request courtesy Lukáš Banič.

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:

test/dialect/mysql/test_on_duplicate.py

@@ -62,6 +62,21 @@ class OnDuplicateTest(fixtures.TablesTest):
                 [(1, "ab", "bz", False)],
             )
 
+    def test_on_duplicate_key_update_null(self):
+        foos = self.tables.foos
+        with testing.db.connect() as conn:
+            conn.execute(insert(foos, dict(id=1, bar="b", baz="bz")))
+            stmt = insert(foos).values(
+                [dict(id=1, bar="ab"), dict(id=2, bar="b")]
+            )
+            stmt = stmt.on_duplicate_key_update(updated_once=None)
+            result = conn.execute(stmt)
+            eq_(result.inserted_primary_key, [2])
+            eq_(
+                conn.execute(foos.select().where(foos.c.id == 1)).fetchall(),
+                [(1, "b", "bz", None)],
+            )
+
     def test_on_duplicate_key_update_preserve_order(self):
         foos = self.tables.foos
         with testing.db.connect() as conn: