Perform additional retrieval of correct column names

Added a workaround for a MySQL bug #88718 introduced in the 8.0 series,
where the reflection of a foreign key constraint is not reporting the
correct case sensitivity for the referred column, leading to errors during
use of the reflected constraint such as when using the automap extension.
The workaround emits an additional query to the information_schema tables in
order to retrieve the correct case sensitive name.

Fixes: #4344
Change-Id: I08020d6eec43cbe8a56316660380d3739a0b45f7

doc/build/changelog/unreleased_12/4344.rst

@@ -0,0 +1,10 @@
+.. change::
+   :tags: bug, mysql
+   :tickets: 4344
+
+   Added a workaround for a MySQL bug #88718 introduced in the 8.0 series,
+   where the reflection of a foreign key constraint is not reporting the
+   correct case sensitivity for the referred column, leading to errors during
+   use of the reflected constraint such as when using the automap extension.
+   The workaround emits an additional query to the information_schema tables in
+   order to retrieve the correct case sensitive name.

lib/sqlalchemy/dialects/mysql/base.py

@@ -733,6 +733,7 @@ def process(element, compiler, **kw):
 
 """
 
+from collections import defaultdict
 import re
 import sys
 import json
@@ -1979,6 +1980,10 @@ def initialize(self, connection):
 
         default.DefaultDialect.initialize(self, connection)
 
+        self._needs_correct_for_88718 = (
+            not self._is_mariadb and self.server_version_info >= (8, )
+        )
+
         self._warn_for_known_db_issues()
 
     def _warn_for_known_db_issues(self):
@@ -2130,8 +2135,56 @@ def get_foreign_keys(self, connection, table_name, schema=None, **kw):
                 'options': con_kw
             }
             fkeys.append(fkey_d)
+
+        if self._needs_correct_for_88718:
+            self._correct_for_mysql_bug_88718(fkeys, connection)
+
         return fkeys
 
+    def _correct_for_mysql_bug_88718(self, fkeys, connection):
+        # Foreign key is always in lower case (MySQL 8.0)
+        # https://bugs.mysql.com/bug.php?id=88718
+        # issue #4344 for SQLAlchemy
+
+        default_schema_name = connection.dialect.default_schema_name
+        col_tuples = [
+            (
+                rec['referred_schema'] or default_schema_name,
+                rec['referred_table'],
+                col_name
+            )
+            for rec in fkeys
+            for col_name in rec['referred_columns']
+        ]
+
+        if col_tuples:
+
+            correct_for_wrong_fk_case = connection.execute(
+                sql.text("""
+                    select table_schema, table_name, column_name
+                    from information_schema.columns
+                    where (table_schema, table_name, lower(column_name)) in
+                    :table_data;
+                """).bindparams(
+                    sql.bindparam("table_data", expanding=True)
+                ), table_data=col_tuples
+            )
+
+            d = defaultdict(dict)
+            for schema, tname, cname in correct_for_wrong_fk_case:
+                d[(schema, tname)][cname.lower()] = cname
+
+            for fkey in fkeys:
+                fkey['referred_columns'] = [
+                    d[
+                        (
+                            fkey['referred_schema'] or default_schema_name,
+                            fkey['referred_table']
+                        )
+                    ][col.lower()]
+                    for col in fkey['referred_columns']
+                ]
+
     @reflection.cache
     def get_check_constraints(
             self, connection, table_name, schema=None, **kw):

test/dialect/mysql/test_reflection.py

@@ -4,7 +4,7 @@
 from sqlalchemy import Column, Table, DDL, MetaData, TIMESTAMP, \
     DefaultClause, String, Integer, Text, UnicodeText, SmallInteger,\
     NCHAR, LargeBinary, DateTime, select, UniqueConstraint, Unicode,\
-    BigInteger, Index
+    BigInteger, Index, ForeignKey
 from sqlalchemy.schema import CreateIndex
 from sqlalchemy import event
 from sqlalchemy import sql
@@ -647,6 +647,50 @@ def test_non_column_index(self):
             [{'name': 'foo_idx', 'column_names': ['x'], 'unique': False}]
         )
 
+    @testing.provide_metadata
+    def test_case_sensitive_column_constraint_reflection(self):
+        # test for issue #4344 which works around
+        # MySQL 8.0 bug https://bugs.mysql.com/bug.php?id=88718
+
+        m1 = self.metadata
+
+        Table(
+            'Track', m1, Column('TrackID', Integer, primary_key=True)
+        )
+        Table(
+            'Track', m1, Column('TrackID', Integer, primary_key=True),
+            schema=testing.config.test_schema
+        )
+        Table(
+            'PlaylistTrack', m1, Column('id', Integer, primary_key=True),
+            Column('TrackID',
+                   ForeignKey('Track.TrackID', name='FK_PlaylistTrackId')),
+            Column(
+                'TTrackID',
+                ForeignKey(
+                    '%s.Track.TrackID' % (testing.config.test_schema,),
+                    name='FK_PlaylistTTrackId'
+                )
+            )
+        )
+        m1.create_all()
+
+        eq_(
+            inspect(testing.db).get_foreign_keys('PlaylistTrack'),
+            [
+                {'name': 'FK_PlaylistTTrackId',
+                 'constrained_columns': ['TTrackID'],
+                 'referred_schema': testing.config.test_schema,
+                 'referred_table': 'Track',
+                 'referred_columns': ['TrackID'], 'options': {}},
+                {'name': 'FK_PlaylistTrackId',
+                 'constrained_columns': ['TrackID'],
+                 'referred_schema': None,
+                 'referred_table': 'Track',
+                 'referred_columns': ['TrackID'], 'options': {}}
+            ]
+        )
+
 
 class RawReflectionTest(fixtures.TestBase):
     __backend__ = True