Calling .update() on the virtual "pk" field for composite primary keys raises AttributeError

[Jacob Walls]

Do we want to support .update(pk=... for composite primary keys?

If not, let's raise a friendly error.

Rough test for demo purposes:

tests/composite_pk/test_update.py

@@ -175 +175 @@
 
         with self.assertRaisesMessage(ValueError, msg):
             Comment.objects.update(user=User())
+
+    def test_update_lhs_pk(self):
+        qs = Comment.objects.filter(user__email=self.user_1.email)
+        qs.update(pk=(1, 100))  # supported?

---

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ERROR: test_update_lhs_pk (composite_pk.test_update.CompositePKUpdateTests.test_update_lhs_pk)
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Traceback (most recent call last):
  File "/Users/source/django/tests/composite_pk/test_update.py", line 182, in test_update_lhs_pk
    qs.update(pk=(1, 100))  # decide whether to support this or not
    ~~~~~~~~~^^^^^^^^^^^^^
  File "/Users/source/django/django/db/models/query.py", line 1254, in update
    rows = query.get_compiler(self.db).execute_sql(ROW_COUNT)
  File "/Users/source/django/django/db/models/sql/compiler.py", line 2068, in execute_sql
    row_count = super().execute_sql(result_type)
  File "/Users/source/django/django/db/models/sql/compiler.py", line 1610, in execute_sql
    sql, params = self.as_sql()
                  ~~~~~~~~~~~^^
  File "/Users/source/django/django/db/models/sql/compiler.py", line 2044, in as_sql
    values.append("%s = %s" % (qn(name), placeholder))
                               ~~^^^^^^
  File "/Users/source/django/django/db/models/sql/compiler.py", line 568, in quote_name_unless_alias
    r = self.connection.ops.quote_name(name)
  File "/Users/source/django/django/db/backends/sqlite3/operations.py", line 190, in quote_name
    if name.startswith('"') and name.endswith('"'):
       ^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'startswith'

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